In all fairness, the compaction is a major pain in Mongo. I get a little worked up about this because I cant think of another database that handles compaction this poorly, but feel free to correct me if Im wrong.
Have you tried turning on power of 2 allocation?
In general, it makes compaction much less important.
Though online compaction is definitely needed.
Yes we switched to it a month ago, which improved it but like you said, we are still having to compact frequently and having the hassle of switching the primary. Cassandra has turned out to be much more performant and easier to maintain for our use case.