Ok, first of all, turn off the I/O Safety Buffer. This adds latency but provides no useful benefit unless your machine is really struggling with considerable audio artifacts e.g. pops, crackles etc. Also, are you getting artifacts with that internal audio config? If not, this will confirm that the problem isn’t with your machine itself.
Regardless, with your UA Apollo, you should definitely NOT be experiencing anywhere near the latency you’ve described nor the artifacts you’ve mentioned. Something is wrong, especially if you’re not getting audio artifacts with the internal audio. With the UA Apollo, you should be getting amazing performance and audio quality. So this leads me to wonder if there’s something not quite right with either your macOS installation or UA driver installation. With MainStage, you really need to run a very tight ship in terms of system optimisation if you want exceptional performance, unless you have an Apple Silicon machine…..which are unbelievably efficient.
Do you have enough HDD space to create a new system container and do a fresh install of macOS for testing? A better option would be to forego this and just do a clean install of your drive anyway but if you have the space, creating and setting up a new container is a great way to test things. You can simply boot into either volume when starting/restarting your machine.
2020 MacBook Pro 16", 2.4 GHz 8-Core Intel Core i9, 64GB RAM, 2TB SSD, OSX 11.2.3 RME Fireface UFX+
MainStage v3.4.4 / v3.5.3