Fluctuating GFR after transplant

Hi all- my husband (46, underlying condition was fsgs) was extremely fortunate to receive a living donation from family in 2021. He's been doing well but out of nowhere his GFR dropped from 54 to 29 in the space of 6 weeks, then rebounded up to 46, then back down to 36 in a week, then back to 46 last week. His creatinine has been anywhere from 2.2 to 1.8 during that time, and his potassium skyrocketed to 5.9 (is now down to 4.6). He has been good on taking meds and hydrating. His kidney tissue looked fine on biopsy, nuclear scan didn't show anything. We have no answers as to why this happened or how to prevent a recurrence, and most importantly, what this means for the lifespan of the transplant. I'm wondering if anyone has had similar experiences and has any info about causation or long term outcomes? We have planned our future on him staying healthy for several more years and if that's not going to be the case we want to start hitting some travel goals before traveling becomes too challenging. We are just trying to gather as much information as possible to make the best of the time we have. Thanks.