I've done a lot of those TCS decoders, you need to cut the tabs on the motor and only leave a small enough amount to solder on to, then once you solder the wires, you need to put some kapton/electrical tape over the contacts to isolate it from the chassis, making sure to cover the tabs and brush caps.I also take a small file and file a small groove on the side of the motor for the wire the run up the side. It doesn't need to be very deep. Also when you mill the frame, you need to mill along the top for the wires to sit down level with the top of the chassis or the shell won't sit properly. Id say that your decoder was shorting out from the motor not being isolated from the chassis. TCS has an installation guide for this on their website

Looks good, all the best at getting them to run have great confidence in you to do it as usual. Thanks for sharing, glad to see yo again 😀