I'm not a fan SPAs for everything but something like notion has to be a SPA. I don't see how prioritizing backend frameworks as the most important part helps here.
OP asked for a full stack solution so that's what I gave him. There's no reason you can't use rails and something like react on the frontend for building a SPA Notion clone. Rails may seem like overkill for an API-only mode but I think it's perfectly useable and pleasant to develop that way.
Just did a quick google search and this is the first result I found: https://github.com/brandonfang/lilnotion