The rear seat belts in my 2018 GMC do not retract all the way any more, and I believe it's due to the retractors. Covert wants a $978 to replace both of them. I'm not sure if this is a replacement I'm willing to take on myself or not, mostly due to having to remove the trim and such to access the seat belts.

Any thoughts? Is this a safety issue if they don't retract all the way?

You can buy entirely new seatbelts for less than that. I have an old Land Cruiser that had lap belts in the back. I purchased retractable ones from the site below and installed them after watching a YouTube video. I understand your concern about not wanting to do it yourself, but it is relatively easy to do.

https://www.seatbeltsplus.com/?gad_source=1

It's definitely a safety hazard if they don't fully retract.

Typically I've found the retractors are not the problem. Rather the seat belts are very dirty and prevent the retractors from working properly.

I've had to have a car detailer shampoo and clean those belts and let them dry. You have to pull it all the way out and clamp it somehow to do this. But this has fixed it.

I see this primarily in pickups or work trucks where there's been a lot of dirty hands.