Hello @gaucho and welcome to 
forum.
You are the first member I have come across who actually got Megasync working. even though as you said you broke it. Mega support technician told me it is not supported on Arch, but it seems I was misinformed since you had it working at one time.
Presently I only use the Mega browser extension, but would prefer to do it the way you had it. So hopefully some genius can chime in and help solve it, so we can all benefit. Have you seen this post on Mega?
Mega does not seem to be very popular here, but since I use it just as a secondary backup for syncing non-sensitive .md files from Obsidian, it’s not a real concern.
Mega is in the AUR