Progressive egg issues with Rhode Island Red

We have a RIR whose recent egg-laying has progressed from large and misshapen eggs with not smooth shells to consistent laying of soft, rubbery shells. This progression has been happening over the last few months. She has been laying nothing but these rubbery-shelled, goopy messes now for a few weeks with no sign of it turning around. Before this, she had been laying consistently great eggs.

I've read about both the Calcium and Vitamind D deficiencies… BUT: She is getting a high-quality feed, regular access to oyster shell and some supplementing of diary to help with any calcium deficiencies. She gets outdoor time with sun exposure and has plenty of room in the run.

No signs of stress. She is eating and drinking normally and never appears lethargic. No drainage from any part of her body, no apparent respiratory issues, no other lesions, healthy-looking comb. Though she is not the alpha female, she doesn't get picked-on by any others. She doesn't appear to strain when laying but does try to regularly lay and will stay in the coop awhile trying. Yesterday she actually laid what looked like two rubbery-shelled eggs back-to-back. No blood or noticeably runny stool. Her vent looks healthy and normal. She did just complete molting a few weeks ago, but so did all the others and their egg-laying is fairly normal now.

No egg issues with our other three hens. All appear to be healthy.
I'm at a loss here and the bird vet wants $110 just to see her. Any ideas on what we could be dealing with? Many thanks in advance!!