The rim gets nearly so hot that you can hardly touch it anymore.
Something is rubbing. MUE. If you drive your car at night, you will even (most likely) see the guilty drum glow orange in the dark.
Until you take the drum off (
when cold please!!)...everything is a guess. Any idea the last time you changed your rear shoes or checked them? I check mine annually.
Could even be some thing wedged in there, keeping the shoes too far spread. As you have used you car like this, (hopefully not too much) you might have worn the one shoe a lot.
have seen shoes worn down to the metal. Drums can warp. Hot ones warp faster.
I have 100,000 kms on my rear brake cylinders. No issues, though the earlier (pre-June 1993) Girling type can fail at any time.
Rear brake cylinders, either kind, do NOT have to be replaced as a set. Nothing gained in doing so..just carry an extra. If you want to have a forever one, just send the replaced one
off to PastParts or Classic Car Brakes and have them refurbish and sleeve them in stainless steel. Most car parts can be made to last forever, but owners must do this. After all,
manufacturers certainly don't want to make "forever" parts.

Good luck!
gmg
P.S. BTW, I was told by the former service manager (2003) and then developer at the Works when we sorting my current (2003) that the Production Department was installing.
adjusting the rear brakes (CAPAROs June 1993 on) in the wrong way. This prejudiced the hand brake function and the ability of the self-adjusting brakes to self-adjust.
It is
one of the reasons UK racer prefer the Girling rears when it comes to this feature as they are manually adjusted. Me too..but I loyally stick with my CAPAROs..and who can find the
correct Girling backplates any more? (for a road car, we are merely swapping problems)
I wrote an article on how to cure the issue.
https://www.gomog.com/allmorgan/APBRAKES.html#SAFETY