Thanks for the advice. Another question if I may. Does trimming consist of
cutting off some of the roots entirely, leaving others intact? Or is it
cutting all the roots back, to (say) half their original length?

On a good note, my Massdevalia is about to bloom!

I would trim any mushy roots back until you find good healthy root tissue.
But I wouldn't completely cut off healthy roots just to make the plant
easier to repot, if that's what you mean. I tend to trim them evenly, as do
most of the growers around here. The plant doesn't need five miles of
roots! Whether that means half or not depends on how much you have to begin
with.

Diana