But the implementation of Gerrit seems rather unloved, it just seems to get the minimal maintenance to keep Go/Android chooching along, and nothing more.
There are lots of people who are very fond of Gerrit, and if anything, upstream development has picked up recently. Google has an increasing amount of production code that lives outside of their monorepo, and all of those teams use Gerrit. They have a few internal plugins, but most improvements are released as part of the upstream project.
My company has been using it for years, it's a big and sustained productivity win once everyone is past the learning curve.
Gerritforge[0] offers commercial support and runs a very stable public instance, GerritHub. I'm not affiliated with them and not a customer, but I talk to them a lot on the Gerrit Discord server and they're awesome.
Gitlab CI is good but we use local (k8s-hosted) runners so I have to imagine there's a bunch of options that provide a similar experience.
Just like with plain git - in GitLab you can merge a branch that has multiple separate commits in it. And you can also merge (e.g. topical/feature) branches into one branch - and then merge that "combined" branch into main/master.
Though most teams/project prefer you don't stretch that route to the extreme - simply because it's PITA to maintain/sync several branches for a long period of time, resolving merge conflicts between branches that have been separate for a long time isn't fun, and people don't like to review huge diffs.
This is how Gerrit operates "natively" - the commit message and everything is part of the artifact under review exactly like the diff.
If the model is to squash an MR into a single commit before merging it, I'd then want to be able to have MRs that depend on each other.