Welcome!
Unfortunately, with the way ESU/Scaletrains designed the motherboard on the rivet counter units, the LM-21-3S (and more specifically and Non-ESU decoders) doesn't play nicely with ScaleTrains 21-pin locomotives. I've spent the last year (on and off) understanding the motherboard in a ScaleTrains GP30 and I know why the LM doesn't work. I have not been able to figure out a way to make it work without replacing the ScaleTrains motherboard. DecoderBuddy is the best route I know of at this point to replace the motherboard.
Also, read this thread where Kevin has some more information on the 21-pin with a ST loco:
https://rpug.pdc.ca/index.php/topic,1666.msg11645.html#msg11645I can't speak to the load balancing as I use RailPro on my small (in progress) switching layout, although from what I have read it works well and can be used with DPU's.
The lighting effects with the LEDs are really great and mimic the prototype extremely well. Also, if a lighting option doesn't exist, Ring is available to help.