I agree that GMs often nerf illusion making it hard to play semi useless.
I honestly feel like Going cleric is conflict to "he is an Great old One trapped in a mortal vessel" it does matter if its in his head or not, being a cleric implies following a Deity out side your self.
I would consider Adjuration at level 2 based on your stated goals something like this:
Wizard skills: Arcana and Investigation Background (Cloistered Scholar): History and Religion to prove you are the he is an Great old One trapped in a mortal vessel and the recall his lost memories by searching that information... real or not.
Point buy STR: 9 | DEX: 13+1 Deep Gnome =14 | CON: 14 | INT: 14+2 Deep Gnome = 16 | WIZ: 14 | CHA: 8
Cantrips:
Toll the Dead (Necromancy ,60ft Wisdom Save great against non-druid/Monk/Cleric/Rangers/scouts AND as a save spell it does not have disadvantage vs enemies in melee)
Frostbite (Evocation, 60ft constitution save, because its a CON save it weaker against Fighters/Barbarians/Sorcerers which is a good contrast to Toll the dead)
Ray of Frost (Evocation,60ft "to hit" for when you want to target AC because for some reason your target has high , because the 10ft reduction in speed means you can kite melee enemies who might try to "geek the mage"
There is a temptation to take Fire Bolt for higher damage roll or Chill touch to stop all those horrible regen enemies however Toll the Dead is generally better damage and Frostbite/Ray of Frost are better consistent defense through offense. I am not saying don't get Chill touch later but hopefully you can pick it up at level 4 without having run into any regenerating enemies before that... maybe keep a torch to burn trolls despite your awesome 120ft dark vision. After that, future cantrips like mage hand and prestidigitation, and minor illusion (as a noise distraction, despite GM that is specifically listed and their is no verbal to this spell, so if your hidden it works) are good choices.
1st Level Spells:
Absorb Elements (Adjuration, Awesome defense)
Mage Armor (Adjuration, Awesome defense)
Shield (Adjuration, Awesome defense)
Protection from Evil and Good (Adjuration, can be a "have or die spell" but if your don't run across vampires, liches, or ghosts trying to possess you … when does that happen?)
Detect Magic (Divination, Ritual, essentially part of a wizards expected job)
Chromatic Orb (Evocation ,90ft range, Requires a diamond worth at least 50 gp which the spell does not consume, descent scaling damage and it lets you always have the right elemental damage for the job. Fighting a Red Dragon use ice, Fighting a White dragon use fire, need to melt a lock off us acid, enemy taking cover in water lighting)
Alternately, drop Chromatic Orb for Identify (wizard job) or Find Familiar (its not a real bird so it doesn't sleep and can keep watch over you and the party while you sleep and scout making better than the alarm spell.
This means you will have multiple spells to activate/recharge your Arcane Ward which you will like use if you would need it but not waste spells on if your not being attacked. With Mage Armor + Shield you have a 20 AC with temporary Hit points a 14 con means more HP and better concentration saves. Your likely going to want to avoid anything but cantrips for attack and keep spell slots for defense until you get second level spells at level 3 which is why I would consider dropping Chromatic Orb to remove the temptation. Its a good spell but if you want defense focus on defense. Toll the Dead will make you relevant in damage and Frostbite can let you save allies.
The Think it with this build you can just play after 2nd level with the exception of Counterspell, Dispel magic, and Banishment which you will want to pick up. Glyph of Warding, Magic Circle, and later Symbol are good out of combat spells to recharge the Arcane Ward, as well as being just good fun. You should have what you need to survive. You can multi-class if you want, but if you get elven armor you will have an AC17 + 5 shield for 22AC which is tank level in most campaigns and you have other defensive spells that they don't get. I don't see the need to multi-class for defense and that is in disregard to subclass. Keeping range is your best defense and slowing your progression in wizard with a multi-class will hurt your power levels for damage, ASI for increased INT which is better to hit and save DC. I wouldn't worry about feats until you have INT 20 because the higher hit and DC will mean taking enemies out of combat faster, then spell sniper is actually the best defense if you have room for range and Resilient CON without range because it scales and it a static bonus making it more reliable overall also aiding you against a number of other save effects.
ASI lvl 4: INT18 , ASI lvl 8: INT16 , ASI lvl 12: Spell sniper , ASI lvl 16: Resilient CON
That would be my best advice based on your goals. If you want to go Evocation School.. you can, but your best defense there is building the same to start and focusing on multi-target damage to knock down as many enemies as you can as fast as you can.