I'm a big fan of SMC Pentax-A 645 lenses on the GFX 50R/S cameras.
There's a thread over at DPReview that you might find useful:
https://www.dpreview.com/forums/thread/4456465
It's focused on options for tilt-shift, but lenses that are good for that purpose will be just as good or better used a "straight" lenses.
A few posts in I provide information, including sample pictures and sample RAFs, from the Pentax-A 645 45-85mm f/4.5, which I consider to be an excellent lens on the 50R. Judge for yourself: there are various RAFs you can download and explore.
Other Pentax-A 645 lenses I like are listed further down in the thread. Another poster in the thread also mentions the Pentax 67 lenses, e.g., the 55/4 (latest version) is really excellent on a 50R (but I found it to be a bit too big and heavy).
And as the previous poster noted, you can find excellent information and user reviews at Pentax Forums (
https://www.pentaxforums.com/lensreviews/). My only caution is people are offering reviews based on all kinds of cameras, from actual Pentax 645 film cameras through to Pentax 645 digital (D and Z), and various mirrorless.