- Arcane Eye: There would be no cost to constantly scrying on everything, entirely removing the relevance of any other scouting method.
As long as all you want to be scouting is within 30ft. Not really scouting.
- Arcane Gate: There would be no cost to constantly teleporting to every location you can see, removing the relevance of walking.Lets go one by one!Arcane Eye is effective only up to 30ft.
500ft per 10 minutes (600 seconds). That's less than a foot a second and probably won't know what you're teleporting into.
Banishment: This might fare a bit better, but any creature in the campaign that you can watch for 10 minutes can wind up *poofed* to another plane for a minute with no cost? This becomes the most effective way to remove any sentinels or guards.
The creature has to be within your 60ft range for the ten minutes it takes to cast the ritual. It gets a save. It returns within a minute. You have to know about the creature ahead of time so that you can get something distasteful to it.
Clone: This might fare OK, but risks Infinite Clones (or as many as you can afford anyway), if there's no slot cost.
Even if it weren't a ritual, it risks infinite clones (or as many as you can afford anyway).
ConjureSpells (all): INFINITE ALLIES.
You can cast only six of these spells in an hour (the duration of the spell), so you can have only six of these up at a time and if you lose your concentration against one, you are very likely to lose concentration against all of them.
Etherealness: There would be no cost to always being ethereal, making this the best scouting method.
If you are going to use this to scout, remember that you've got a 60ft visual range. Which means that any enemy you stumble across is likely to see you long before you see them. That makes it not the best scouting method.
Limited by how many 5000gp diamonds you can find and this is a ninth level spell. Infinite Portals seems well within the acceptable range for an archmage.
Glyph of Warding: INFINITE TRAPS.
If you want to waste your entire day moving at an exceptionally slow pace.
Legend Lore: This might fare OK, but there would be little cost to learning information about the object, making this the best information-gathering method as long as you had the gp.
Only of objects which are of legendary importance. You want to know about the local thieves' guild? Or some minor temple in the backwoods of nowhere? Legend Lore won't help you there.
Leomond's Secret Chest: This might be OK, but you can squirrel away as many chests as you want for free.
Not really seeing the problem.
Faithful Hound: INFINITE WATCHDOGS
You cast the spell, it is limited to a specific space. What are you going to do, bait all your enemies to a 30ft radius area? Because you can't move more than 100ft away.
Private Sanctum: INFINITE WARDS
Modify Memory: INFINITE NEW MEMORIES
You are going to have to incapacitate the target without fighting it (unless you don't care about it rolling its save at advantage). If the creature realizes what that you are casting a ritual and you are not a friend, it isn't going to hang around for you to finish.
Hallucinatory Terrain: INFINITE ILLUSIONS
A very specific kind of illusion. Again, not seeing the problem here.
Sending: Becomes the best way to communicate, because hey free telepathy.
Again, not really seeing the problem here.
Simulacrum: INFINITE SIMULACRA (well, as many as you can afford)
But, you can do the same thing now without using a ritual
At 1000gp a pop, besides are you really going to go through a dungeon at one foot per minute?
Teleportation Circle: INFINITE TELEPORTS
And..?
Trap the Soul: See Banishment
I don't have the details of that spell on hand, but I believe most of my responses to your comments on Banishment apply.
True Seeing: Become the best scout.
The creature still has to get to where they can see with true sight. That prevents them from becoming the best scout. It doesn't give them stealth, insight, perception of things which aren't hidden (small things or the like), investigation, knowledge skills, etc. So, not really the best scout.