I think you're somewhat missing the point - if the code from A and B only works if joined with C, then you should squash them all into one commit so that they can't be separated. If you do that then the problem you're describing goes away since you'll only be rebasing a single commit anyway.

Whether this is valuable is up to you, but IMO I'd say it's better practice than not. People do dumb things with the history and it's harder to do dumb things if the commits are self-contained. Additionally if a feature branch includes multiple commits + merges I'd much rather they squash that into a single commit (or a couple logical commits) instead of keeping what's likely a mess of a history anyway.

That is literally what I advocate you do for the main branch. A feature branch is allowed to have WIP commits that make sense for the developer working on the branch just like uncommitted code might not be self contained because it is WIP. Once the feature is complete, squash it into one commit and merge it into main. There is very little value to those WIP commits (rare case being when you implement algorithm X but then change to Y and later want to experiment with X again).