There's definitely a happy medium. One setting may indeed be too few, but JVM's many options ends in mass cargo-cult copypasta, often leading to really bad configurations.

Tuning options don't work well with diverse libraries, though. If you use 2 libraries and they both are designed to run with radically different tuning options what do you do? Some bad compromise? Make one the winner and one the loser? The best you can do is do an extensive monitoring & tuning experiment, but that's quite involved as well and still won't get you the maximum performance of each library, either.

At least with code hacking around the GC's behavior that code ends up being portable across the ecosystem.

There doesn't seem to really be a good option here either way. This amount of tuning-by-brute-force (either by knobs or by code re-writes) seems to just be the cost of using a GC.

That might be true, but from a language design PoV it isn't convincing to have dozens of GC-related runtime flags a la Java/JVM. If you need those anyway, this might point to pretty fundamental language expressivity issues.