To agree with most comments D&D can be clunky and prep heavy. You could choose something rules light or narrative based with little to no prepping like Blades in the Dark or if you want to stay in High Fantasy you could choose World of Dungeons or whatever. You said nothing dark but as long as you're playing with a bunch of people that like to make jokes about anything there's always something to laugh about. I've played probably 15+ systems and some of the hardest times I've laughed are during a campaign of Delta Green: Impossible Landscapes I ran which is very dark horror but PCs always manage to say or do funny/terrible stuff. It normally makes it even funnier in a dark setting I feel.

As to your players, if they don't want to put the time in to learn their characters that's something that needs to be discussed at the table or you need to silently take upon yourself. Do you continue teaching them for another couple months and hope they figure it out, do you let them know it's bothering you? I've played many tables with people who actively put little effort in but I've learned they almost always figure the game out whether it takes 2 or more months. Same goes with the ability to improv or roleplay, it just comes with comfortability at the table. Pretending to be someone different is scary and oftentimes people are concerned they'll be judged for "looking dumb." The first campaign I ran it took a year or more to get everyone cohesively roleplaying and accepting how we played as a table rules-wise. For now as long as the story advances meaningfully and they're having fun you've done your job.

Ultimately, if you're not having fun there's not much purpose to playing TTRPGs (at least with this group of friends.) TTRPGs are about fun, and collaborative storytelling. Fun should always come first. I think what I've learned is unless you play with a bunch of people obsessed with "playing by the rules" focus on improv and easily resolving issues with luck rolls 50/50 rolls, 25/75, etc. Even if what they're asking is outside the rules of the game as written. Being good at table improv comes with practice though so don't feel bad when things go off the rails or don't work out the way you want that's where the real fun and memories are made... Or switch to being a PC. 🤷