Car barely moves when transmission is put in reverse

You could have a loose input shaft nut under the transmission side cover. The nuts loosen up over time. The first step is to check the transmission fluid pressure with a transmission fluid pressure gauge. It takes a lot more pressure to go into reverse then the forward gears. It takes a thin walled socket to tighten the input shaft nut. Plus a holding tool to hold the input shaft while tightening the nut up. The not has to be torqued so do you want to start by checking the transmission fluid pressure? Or try to see if the nut is loose? Of the second engages and does not slip or set a code in the computer for no second gear. Then it sounds like the second gear clutch piston is holding pressure. Reverse uses the second gear clutch pack since it does not have a separate clutch pack for reverse.

Also, do you have any codes in the computer have you checked yet? Here is a guide how to get codes out of your computer:

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing