No time for this now, there's Kissinger jokes to make!

Bloons TD6 is pretty fun when I played it. It's constantly updated with new content and it's not too grindy (only takes a couple hours to unlock everything).

Seconding btd6, it's hard to beat. Has in depth mechanics and secret buffs

Defense grid

I don't really care for the genre but the only one I liked was Bloons. Its got monkeys.

I play Arknights and have fun, but it sounds like you don't want to play a mobile game. It's pretty reasonable to play as a f2p account. Everything can be cleared by lower rarity 3*, 4*, and 5* operators and 4* operators are easy to get. The 6* gacha operators are good, but they're mainly for quality of life and you can go far with just 4* ops. It doesn't spam you with microtransactions ads or anything. Story is pretty good if you like visual novels. The anime cringe stuff is pretty minimal and the operator designs are really cool and neat.

Maybe Iron Brigade if you want a co-op one and can get multiplayer to work?

Rogue tower looks like fun.

Bloons TD 6!

Pixeljunk Monsters (the first, I've never played the second) is great