See my comment above: a cool feature I am implementing into fire.ci now is that it will trigger a new build every time the main branch moves forward until the safe merge happens. If you have fast builds I don't expect this to take more than 2 builds each time (or you have a huuuuge team working on the repo).
Right now I have a one button rebase operation in gitlab, and if that does work (due to conflicts) then I'm at a situation I want to be in: verifying that my changes integrate cleanly.
Yes, that's the case. However there's also the 'merge this when the build successfully completes' that means you can push one button and walk away. They are also developing features around MRs that 'ripple' in this way, so that when you have several going the rest will continue rebasing+building until they either fail the build (due to changes that are no longer correct), can't be rebased (due to a need for conflict resolution), or finally merge successfully.