Hey Ben,
in your config file you had the following lines
> set audio.realtime-prio 99
> set midi.realtime-prio 99
so I assume you set those numbers via the command-line now.
And your limits.conf sets a maximum realtime-prio of 95 for the audio group:
> @audio - rtprio 95
So you could either set Fluidsynth to a lower prio, or increase the rtprio limit for the audio group.
Cheers,
Marcus