Welcome @kaurashima, I moved your post to this related discussion:
- MGUS Symptoms: What symptoms did you experience?
https://connect.mayoclinic.org/discussion/symptoms-etc-of-mgus/
I did this so you can read previous posts and connect easily with other MGUS members like @pmm @scnana @allstaedt57 @mariannalena and many others. I'd also like to tag @lfevold @juniperjgin @gingerw @gingerw @annapocono @cherylmcg who have experience with MGUS and autoimmune conditions.

@kaurashima, I think you are asking the right questions and should pursue them with both you hematologist and rhematologist. Questions like:
- How do you distinguish which symptoms are related to MGUS vs an autoimmune disorder?
- What steps need to be considered to ensure that treatment for RA doesn't accelerate progression of MGUS?
- What do I need to monitor?
- What do I need to know and didn't think to ask?