Playing as a thief with spells, should I do the WIT tests and switch to wizard or not? I was kind of intrigued by it but don't want to miss out on thief specific things for this playthrough.

IIRC I played as a fighter with spells in 1 (vga), then switched to thief when starting this one. I then put it on hold for a few months, so I'm not 100% sure but could check again. In case this matters.

Only Magic Users truly class change into Wizards - Fighters and Thieves stay Fighters and Thieves even when they get the title of Wizard. You don't have to worry about missing out on class-specific quests like that.

Great, thanks.

The Magic User is also the only class who can trigger the ability to play Wizard's Whirl after becoming a Wizard. There is a workaround I always use, but you need to have Blue Frog Mode unlocked - so it only works after proving you've mastered the game. It's also technically a debug mode cheat. But I never liked that hybrids were arbitrarily locked out of certain class features in a single player, single character adventure regardless.

The workaround is to first become a Wizard the normal way, then change global variable 3 (player class) to 2 (Magic User), ask Aziza about magic games, then after she mentions Wizard's Whirl, change back to 1 for Fighter or 3 for Thief. Click the mouth on yourself and you'll find yourself able to challenge Aziza to Wizard's Whirl as the wrong class - and even learn the charged Flame Dart for winning. Or if you really hate Wizard's Whirl (I don't - it's relaxing, Force Bolt Flurry is the annoying one. Having to do mental algebra every time you want to do anything? Pass.) just set global var 102 to give yourself chargeable fireballs. gvar 101 is for Force Bolts.

Well, there is one thing that changes once you complete WIT. When you get the Reversal spell, the "non-thief" final room changes to a spell-based one, requiring Reversal to be cast. You can get there as a thief, but it isn't the "intended" thief route with the tightrope, so you'll probably be fine.