If you do not mind, can we continue this conversation over email? I still may have some technical suggestions that we can discuss and you may or may not consider. Perhaps something you would not like to post publicly.
Plus, the fact that I see on my USBpcaps the leftover (now rubish) data of the truncated SysEx message may indicate a possible bug. They are not proper MIDI messages. I’d like to send you this debug.
We can report back to this post the final results for other users to see.
BTW – thank you for looking at increasing the buffers, that for sure will help a lot.