There’s waterfall.social which has been made open source (Idr under which license. I think it might be AGPL?) Not sure whether that’s what you mean by “open platform”.

It also has a number of nice features added compared to tumblr, like changing which blog of yours is the “main blog”, having a separate dashboard and following list for each blog in an account, a system to detect uncredited art-re-uploading, etc.

The UX is designed from a user perspective, a lot right. The implementation is from a developers perspective mostly wrong.

It's literally the UX that is wrong. Community and their content is why it's still a thing.

I think maybe OP was talking about the overall UI paradigm. The details are janky as all get out, certainly, but the concepts aren't bad at all.

Kind of like what the mastodon/pleroma network is to Twitter?