It’s not a silver bullet, but here is a useful quote and citation to use when filing a mandamus complaint for a delayed decision on a 290B motion to reopen or reconsider. According to USCIS:

. . . . Q. When should I expect to receive a decision on a motion? A. Although some cases may take longer, USCIS field offices and service centers try to adjudicate motions within 90 days. The AAO strives to complete its review of motions within 180 days from the time it receives a complete case file. . . . .

https://www.uscis.gov/forms/all-forms/questions-and-answers-appeals-and-motions