Pay no attention to the people who don't have kids. You're doing just fine. The fact that you're asking honestly makes you a better parent than many.

Kids, as you well know, are gonna figure out unimaginable ways to get themselves in trouble. I've had to tell more than one of my kids, "I'm gonna let you make all the mistakes you want, and I'm gonna be there to pick you up, but I am not going to let you make permanent mistakes." When it comes to advertising, microtransactions, OnlyFans (yes, OnlyFans), the lesson is "these things exist for one purpose, and that is to separate you from your money as much as possible." If you are paying for something, stop and consider whether you can get a substantially similar thing for free, or at least for a lot cheaper.

From reading your other comments, it sounds like you and the other parents all agreed together to drop the hammer on all the kids at once. That is a good idea, and it's great that you have lines of communication open to your kid's friends' parents. That's going to be important when they're older and driving cars, and having access to intoxicants and mall ninja shit.

To your actual question - I saw someone mention Minecraft, that is a fantastic choice. There are "skins" and shit that can be bought, but the game itself it absolutely fully playable and enjoyable without anything beyond the initial game purchase. Running a private server is pretty easy, and I would recommend it, so that the friend group always has a place they can go where the annoyances of the internet-at-large are excluded. Besides that, a kid who is motivated to modify his own Minecraft server is going to be driven to figure out how to do it, and that kind of skill will be super useful for oh so many things throughout life.

If they like arena combat games, Crossout is pretty fun. World of Tanks is okay, but the grind curve is steep. War Thunder is fun for planes and ships, but I am not a fan of their tank play mechanics. All of those are free to play, yes you can buy stuff, but you absolutely do not have to.

There's a single player game that I have to mention: The Long Dark. Winter survival, and there's also a storyline mode. The storyline is really good, and the map is absolutely vast. While it's not one they would be able to play together, it's a great exploration and survival game, and I would be remiss if I didn't point it out.