Indeed, I'd say your best bang for the buck on that is Major Image. It's a short term spell, but you will have lots of 3rd level spell slots, as opposed to 5th level.
So, for specialist wizards, there's a counter-intuitive consequence to how they learn spells. Their scribe costs for specialty spells is far less...which means you want to pay for them. Use your free levelup spell picks to gain nonspecialty spells. That way you maximize your economic advantage.
Some spells at each level to consider, listed not so much as specific suggestions for your particular character, but for generic utility.
Cantrip:
Shocking Grasp (don't underestimate the value of an at-will spell that can be used to damage something in melee, and remove it's ability to OA yo uas you skedaddle the heck away from it)
1st -
Absorb Energy (Reaction resistance to elemental damage will save your keister in more than one situation)
Shield (Reaction huge AC boost is even better)
Magic Missile (I know, it's kind of boring, but it's autodamage)
2nd -
Levitate (not to be underestimated, though Spider Climb can substitute for it sometimes)
Mirror Image (a low level defense buff that is not Concentration and can negate critcals; priceless)
Shatter (a surprisingly good low level area damage spell)
Suggestion (powerful Will save spell that can remove a target from an encounter)
3rd -
Counterspell (one of the best reactions you'll ever have the chance to make)
Dispel Magic (if you don't take Counterspell, take this)
Haste (one of the best ally-buffs in the game)
Hypnotic Pattern (situationally more devastating than Fireball)
Major Image (will work beautifully with your 'real illusions' power)
Slow (absolutely horrific debuff to inflict, seriously it's one of the worst)
4th -
Fire Shield (has defensive and offensive applications; doesn't require Concentration)
Greater Invisibility (lets you cast spells with relative impunity for most encounters)
5th -
Bigby's Hand (extremely versatile in effect, and quite potent against single targets)
Cone of Cold (beautiful AoE damage; usually fairly easy to target past allies with clever positioning)
Hold Monster (for when you absolutely, positively need one enemy to die from autocrits from your allies...which is always)
Passwall (was named "Rary's Tired of The Thief Breaking His Lockpicks" in previous editions; seriously though, it's basically a win button against most obstacles)
Wall of Stone (my favorite Wall spell, because of its versatility, but Wall of Force is also very good)
6th -
Disintegrate (Wall of Force? No problem! Wall of ANYTHING? No problem! Horrible monster with low Dex save? NO PROBLEM! Note - this spell works GREAT in combination with Hold Monster)
True Sight (screw you, illusions!)
Sunbeam (screw you, undead!)
Mass Suggestion (screw you, twenty ogre-trolls we don't feel like fighting today!)
7th -
Etherealness
Magnificent Mansion (free short rest)
Teleport (uh, are we getting TPKed? We are? Okay, everyone hold hands)
8th -
Antimagic Field (it is truly hilarious the amount of horrible, horrible BS this spell stops cold)
Mind Blank (This too)
Dominate Monster (pound for pound, probably the best single-target attack spell in the game; not only instantly defeat a foe, but RECRUIT a foe)
Sunburst (no, really, screw you ENTIRE ROOM full of undead!)
-----------
So, when selecting spells, bear in mind that you are going to refresh your spell slots every WEEK, with one small refresh after a day of rest. You absolutely need to make sure that every spell you cast is as effective as it possibly can be. It's okay to have spells like Clone and Glyph of Warding in your spellbook if you want to pay money for them, but I wouldn't prepare them to cast on a regular basis. It's wise therefore to have at least one or two spells per level that you will always want to have prepped.
To this end, I'd suggest reassessing your 7th and 8th level spell choices. None of those are likely to be useful in the dungeon at any given moment. You can always use the slots to upcast lower level spells, but that's often a very inefficient way to use slots.
Spells like Demiplane and Mirage Arcane and Glyph of Warding are also very specialized in application...you may not be getting a lot of use out of them (though I guess if you want an emergency resurrection clone stashed in a demiplane, that's one way to do it
).
So, for specialist wizards, there's a counter-intuitive consequence to how they learn spells. Their scribe costs for specialty spells is far less...which means you want to pay for them. Use your free levelup spell picks to gain nonspecialty spells. That way you maximize your economic advantage.
Some spells at each level to consider, listed not so much as specific suggestions for your particular character, but for generic utility.
Cantrip:
Shocking Grasp (don't underestimate the value of an at-will spell that can be used to damage something in melee, and remove it's ability to OA yo uas you skedaddle the heck away from it)
1st -
Absorb Energy (Reaction resistance to elemental damage will save your keister in more than one situation)
Shield (Reaction huge AC boost is even better)
Magic Missile (I know, it's kind of boring, but it's autodamage)
2nd -
Levitate (not to be underestimated, though Spider Climb can substitute for it sometimes)
Mirror Image (a low level defense buff that is not Concentration and can negate critcals; priceless)
Shatter (a surprisingly good low level area damage spell)
Suggestion (powerful Will save spell that can remove a target from an encounter)
3rd -
Counterspell (one of the best reactions you'll ever have the chance to make)
Dispel Magic (if you don't take Counterspell, take this)
Haste (one of the best ally-buffs in the game)
Hypnotic Pattern (situationally more devastating than Fireball)
Major Image (will work beautifully with your 'real illusions' power)
Slow (absolutely horrific debuff to inflict, seriously it's one of the worst)
4th -
Fire Shield (has defensive and offensive applications; doesn't require Concentration)
Greater Invisibility (lets you cast spells with relative impunity for most encounters)
5th -
Bigby's Hand (extremely versatile in effect, and quite potent against single targets)
Cone of Cold (beautiful AoE damage; usually fairly easy to target past allies with clever positioning)
Hold Monster (for when you absolutely, positively need one enemy to die from autocrits from your allies...which is always)
Passwall (was named "Rary's Tired of The Thief Breaking His Lockpicks" in previous editions; seriously though, it's basically a win button against most obstacles)
Wall of Stone (my favorite Wall spell, because of its versatility, but Wall of Force is also very good)
6th -
Disintegrate (Wall of Force? No problem! Wall of ANYTHING? No problem! Horrible monster with low Dex save? NO PROBLEM! Note - this spell works GREAT in combination with Hold Monster)
True Sight (screw you, illusions!)
Sunbeam (screw you, undead!)
Mass Suggestion (screw you, twenty ogre-trolls we don't feel like fighting today!)
7th -
Etherealness
Magnificent Mansion (free short rest)
Teleport (uh, are we getting TPKed? We are? Okay, everyone hold hands)
8th -
Antimagic Field (it is truly hilarious the amount of horrible, horrible BS this spell stops cold)
Mind Blank (This too)
Dominate Monster (pound for pound, probably the best single-target attack spell in the game; not only instantly defeat a foe, but RECRUIT a foe)
Sunburst (no, really, screw you ENTIRE ROOM full of undead!)
-----------
So, when selecting spells, bear in mind that you are going to refresh your spell slots every WEEK, with one small refresh after a day of rest. You absolutely need to make sure that every spell you cast is as effective as it possibly can be. It's okay to have spells like Clone and Glyph of Warding in your spellbook if you want to pay money for them, but I wouldn't prepare them to cast on a regular basis. It's wise therefore to have at least one or two spells per level that you will always want to have prepped.
To this end, I'd suggest reassessing your 7th and 8th level spell choices. None of those are likely to be useful in the dungeon at any given moment. You can always use the slots to upcast lower level spells, but that's often a very inefficient way to use slots.
Spells like Demiplane and Mirage Arcane and Glyph of Warding are also very specialized in application...you may not be getting a lot of use out of them (though I guess if you want an emergency resurrection clone stashed in a demiplane, that's one way to do it
).
