If you architect your protocol cleanly, you should be able to run the simulation client side too without much effort (certainly the web platform has everything you'd need). This is how modern game engines do it, it goes back to the Quake VM and probably beyond.

You'd still get multiplayer "for free", but it could be turned on and off. You could do it with zero code change for the actual games, they don't have to know.

It seems some of the games could do with a singleplayer mode that doesn't depend on the backend having free slots.

Just an idea from a fellow web games person!

as a matter of ux, users probably don't need to know that a session is being instantiated, and probably expect a simple play button

fun

I was obsessed with the Arcade Game Construction Kit on the C64.

I worked on a similar concept (padgames.io now defunct) that offered a state sync networking system with rollback netcode style stuff. It could handle exposing only certain information or fake info to certain clients (to prevent cheating). It integrated super nicely with vuex or react stores as it was all observable and all game actions just turned into state mutations.

I made the game I wanted, enjoyed it with family and friends, and then let COVID sweep it away. Congrats on continuing to work on it.