IMO, as others have said before me, the only winning move is not to play. I like the advice in these threads:

Look at their spell list. Pick 3-4 spells they are most likely to cast, as they will not be casting more than 5 for a regular combat. Print spell cards for those. If you happen to cast something else when appropriate situation calls for it, you can look up few of those.

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Yeah, you do have to read through the spell section once from top to bottom if you don't know what spells do. If you want to wing it, you just need to have an idea of what the spell does. Then you can remember that, and turn to the spell to bring yourself back up to speed on the mechanics.

https://www.enworld.org/threads/npc-caster-kits.436296/

https://www.enworld.org/threads/how-do-you-keep-track-of-spells-for-multiple-npcs.660693/

One big problem that the D&D system has always had is that if you throw enough save or suck spells at a PC, they'll eventually fail a save. For example, seven 1st level spellcasters each casting 'Charm Person' has a very high chance to charm most PCs.

The second thing I would suggest to you is even if you find the situation requires it, don't try to track NPC spells like you would if they were PCs. The work load will be overwhelming.

Instead, list how each NPC begins the fight buffed (or not in the event of surprise), and then list roughly as many spells as the NPC will cast before dying in roughly the order they'll cast them if opportunity arises. This will mean you only need to track 3-4 spells. Then you're tracking much shorter lists.

If for some reason you get off script and the NPC finds themselves in a situation where they need some other spell, simply decide that they have it and add it to the list of spells. Rarely when you wing it like this are you going to differ much in what happens than if you choose all the spells ahead of time and were crossing them off, and the whole approach can be justified by the fact that most NPC spellcasters are geniuses who would have prepared the best and most useful spells available in the first place. Remember, the players are unlikely to be tracking the NPCs that carefully either.

As for knowing what spells do, it's the duty of a GM to know the system at least as well as anyone else at the table. If you can't wing a spell, then don't - cast a spell you can wing. Otherwise, make sure you've prepped ahead for the spells you are likely to use. Spell note cards are something I recommend for all PC spellcasters, and they can be handy in situations like this when you are running NPCs as well.