Okay, I increased vm.min_free_kbytes to 16384 in /etc/sysctl.d/osmc.conf and rebooted the Pi. I’ll let you know how the system performs.
Update 1: Upping to 16384 kb didn’t help. Now testing with 20480 kb.
Update 2: No, 20480 kb was no good, either. Switching back to 16384 and setting vm.swappiness to 80.
Update 3: That also did not help.