I’ve been using Concourse CI and it’s fairly light weight and has great declarative pipelines. Learning curve is not too steep and it has a command-line tool to update your pipelines. Maybe it’s a stupid suggestion, but you could connect the worker in that setup through ssh to a Mac for building, but of course that takes away some of the comfort, but you will have to run a dedicated build machine anyway.
Also, doesn’t Travis CI have macOS support ?