One idea for Marge-bot (don't know if you already do this):
Rust-lang has their bot (bors) automatically group together a few merge
requests into a single merge commit, which it then tests, then, then the
tests pass, it merges. This could help reduce CI runs to once a day (or
some other rate). If the tests fail, then it could automatically deduce
which one failed, by recursive subdivision or similar. There's also a
mechanism to adjust priority and grouping behavior when the defaults aren't
sufficient.

Jacob