Have you heard of Obsidian.md? It’s not open source but it has hundreds of powerful plugins that are and give them much more power and versatility than Notion, as of today. Check the Canvas, DB folders, Projects, Dataview plugins and many others, or the ones related to playing TTRPG within Obsidian (check Josh Plunkett’s YT channel in this regard).
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Obsidian Publish doesnt publish a fully plugin’d out vault as a website, just the base markdown. So if you rely a lot on dataview, Canvas etc, theres not a direct vault-to-website path.
All that said, if you are using mostly the base plaintext markdown, there are free and fairly straightforward ways to a website. I’ve used 2 different methods to push my vault folder with git to github to autogenerate websites.
Here’s a big vault of dungeon23 megadungeon resources, similar to your notion project, that I did this way. https://old.sketchyvanrpg.com/vault/