The annual chance of MGUS progressing to MM depends considerably more on the MGUS risk level (low, intermediate, high) than on any "outside factors". There is no quick way to describe the assessment of risk level. It depends on multiple tests, frequently done at different times (to show variation in results, if any), as professionally interpreted (preferably by a hematologist / oncologist). If you want to get into the recent weeds (may require subscription for full text):
https://jamanetwork.com/journals/jamainternalmedicine/article-abstract/2830017