FoundryVTT. It's a one time license fee and though the code is not completely open source, it's mostly just client side JavaScript that is completely unobfuscated and supposed to be able to be viewed so everyone can modify it with addons. The server is obfuscated JavaScript.
It's completely and easily self-hostable.
So far, the company/people behind are just TTRPG nerds just wanting a good and friendly VTT (...wanting to make money with it).
It is a full VTT though. If you really only want chat+dice rolling, I would suggest a matrix server with a dice roll plugin/bot.