The drivers side seat belt is slow/reluctant to rewind. Can this be addressed without major effort/pain?

I think the only way to fix that is to replace the belt assembly. That's pretty easy to do. Just make sure you find one from another W126 that'll fit if you go used, since the W126s used about 2 or 3 different belt variations over the years. You can go new to be sure, but the cost alone will be $200+, but you'll have a pristine part with many years left.

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

I have a 1991 300d, does the same thing. Be careful, because they have pyro stuff in the seat belt to stick in place during a crash.

You can take your stuff around it off, which is pretty easy to do, then use an air blower to blow all the lint off from years of rubbing along your clothes when it retracts, then once it's blown off, suck it up with a vacuum cleaner. This might help, but will not cure the problem.

Jeff

Lazy ratractors are a German mainstay from this era. Unlikely you'll find a used set that is much better.

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Gone to the dark side

- Jeff