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.
So its not just 'one button rebase operation' its 'one button rebase and wait for build to finish operation' before it can be reviewed again.