Your remote control sounds very interesting, maybe do an article here on how you did it?
How heavy is it now it’s got different batteries, maybe it just needs more weight? I converting one earlier this year to metal wheels to run on standard 12v metal track with a motor in each bogie and even with 2 driven axles it sat on the spot spinning it’s wheels until I put the original 4x D cells back in it to give some weight, then it went like the clappers!
Another possible cause could be the original rubberised traction wheels have gone hard and aren’t gripping any more. I have replaced most of the drive wheels on my Hymeks with new ones made by ybot999 on eBay. After that they do pull a decent length train. Here’s his current eBay listing: https://www.ebay.co.uk/itm/293759671193?hash=item446570db99:g:b80AAOSwEBhdfldl