If I were starting such a project -- and I must resist the temptation to do so -- I'd start by taking a very close look at Gerrit.

As a technology base to fork from, probably not ideal. But its flows are something to learn from.

The PR process in GitHub has always been garbage, and its cargo cult adoption by the whole industry is sad. But also unnecessary. There were always alternatives, but GH's refusal to do proper multi-round review and its tendency to encourages giant messy merge commits with no ability to track discussions between changes is an organizational nightmare, and now with LLMs it's even more terrifying.

Every company I've worked at since I left Google has had this problem with giant "take it or leave it" submissions. Dozens of commits in one "review". No ability to properly track changes between revisions. A mess of commits that all land at once.

I don't see how one can build a serious software team structure over top of this. It's a mess. And GitLab only makes it slightly better.