RPG

I'm biased against VTTs, maybe I'm wrong.

Ah, your original post didn't mention them... and then asked what was missing. VTTs are pretty clearly the elephant in that room that was omitted.

It sounds like VTTs might not be your jam, which is fine. But to address possible points of confusion...

They always seem to want to sell rulesets and adventures on top of subscriptions (eww) or the software.

Foundry and Fantasy Grounds are both one-time purchases and most (but not necessarily ALL) rulesets are free along with their SRD equivalent if one exists.

Now, both of these do sell optional add-ons, and have third-party folks who build and sell or Patreon-gate stuff they make. A one time purchase does not entitle you to everything that anyone in the world can imagine to build on top of the core VTT... but it does entitle to the core VTT many rule systems and regular software updates for a good long while.

Not sure if you can play a game previously own without buying it again, or sometime they limit the ability to make a custom one without a subscription and so on.

Both Foundry and FG allow you to enter your own content/characters/monsters/items/adventures into existing systems (including generic systems that try with varying degrees of success to be system agnostic), or to make your own system from scratch (though that's a difficult and time consuming programming project... it does enable folks around the world to create their own custom systems and most of those are made available for free for others to use as well).

Now, both these VTTs also sell and allow the additional purchase of adventures and other stuff that was previously published in book form. This is generally sold separately from the book, and from the VTT. These digital adaptations are nice and if you want to pay only for the core VTT and be done with spending money it's easy to get jealous looking at them. But that's very much like complaining about having to pay to see a Marvel movie because you own the comic. These are different things from the book, made by different people, and they're a lot of work to create well. The best publishers like Paizo offer discounts for multi-format purchases, but they need customers to tell them what formats to invest in by buying stuff and they need money to pay people to do the adaptions.

All of which is to say, you may not be into VTTs. They are a whole thing and getting them working well is a major distraction from from normal prep. But with some of them it's certainly possible to develop your own adventures without a subscription or additional purchase beyond the core product, or to adapt a paper/pdf adventure in a supported system. I've done both and enjoyed the process and results.