We just recently migrated from dying Appcenter to a new CICD, and after all the research we decided to go with Fastlane + CI.
For the CI for first month we will try Bitrise.
But the whole pipeline is built in Fastlane.
And we have quite a project. With multiple flavors (We have implemented fully custom since no flavors in KMP), with multiple signing keys for android.
I didn't have lots of issues learning fastlane for KMP and seems like there are Fastlane + your gitlab integration.
So maybe it is a way for you as well, since 0 answers yet.