I think if you're trapped in the Apple ecosystem then Metal is your only production-quality option. But beginner-level learning material for it is quite sparse, so it's better to transition from OpenGL to Metal rather than learning Metal from scratch. (If you're an experienced graphics dev, you should be able to follow up on Metal by just reading their example source code)
There are a few nice books, they are a bit out of date though.
MoltenVK is pretty good I've been told
Nope, the limitations are just too huge for actual usage. Heard Godot Engine tried to use MoltenVK for macOS but experienced too many issues so they just developed a Metal backend.
The fundamental problem is Metal 3 is just too high-level to be able to emulate all of Vulkan's behavior. The new Metal 4 API (which is more low level and similar to Vulkan in many ways) might have improved things recently, but sadly MoltenVK hasn't been rewritten to this new API yet.
Worse than OpenGL though? Haven't had the time to really look into it, probably native Metal is better, yes, but either way it might be easier to port/adapt from Vulkan than from OpenGL (due to similar concepts, no global state, etc)
Yes, hence there is now a new layer, KosmicKrisp.