Well, I'm not a doctor, but some people just stop b12 or methylfolate for awhile, and take methionine by itself. It does convert to homocysteine, so I believe one will eventually need some b12.
Here's an old post on Lynch's site as an example. Look for 'Sero's comment (I disagree though w/Walsh's 'overmethylator/undermethylator' stuff. It's too black and white, but no doubt one's methylation can be over-driven or under-driven for periods. It would be interesting to find out if Sero ever went back on 'some' b12. Also it sounds like he was taking a LOT of extra things, so it's hard to tell if it was caused by too much b12 or folate or (fill in the blank) -- but, having said that, methionine clearly helped him bring down his histamine levels.
p.s. Also note how Lynch pushes back a bit on his comment, but a couple years later added methionine to his list of options if one was having a bad reaction to methylb12 or methylfolate.
From 2011:
https://mthfr.net/taking-folate-and-feeling-badly-methylation-requires-balance/2011/11/15/