Generic army-size fantasy wargames? DIY elements galore?

[Hobgoblin]

-Kings of War. Not technically generic, but nearly every unit and army type is covered in the current Rulebook. Rank and Flank without individual figure removal.

That's a good call. There's also the cut-down Ambush variant for smaller battles (still rank'n'flank), which is essentially the same game with a half-sized table and fewer units - great if you want a very quick game.

[Basementboy]

I’ll also recommend Dragon Rampant- all of the “rampant” games are a lot of fun, very DIY, and have a large enough following and simple enough rules that you can deffo get someone to use them with.

[BZ]

I just got a read quick through of Mike Hutchinsons (Gaslands, Billion Suns) fantasy battle game, Hobgoblin. Its seems to be a very quick game and all the units can be individualized (there is a great online army builder). The magic part has also variety, so hopefully you wouldnt need to make own spells (which, I think would be a problem in any system). Definitely worth a look!