Howdy,
Let me quickly say, that like you I’ve been at this issue for along time. Starting months ago, I’d read plenty of threads, watched a bunch of videos. And I’ve tried the fixes that you have.
As people have mentioned and you’ve probably read, the issue seems to be a problem with variable framerate. (There’s free software called Mediainfo you can download if you’d like to see what the minimum and maximum framerate is for the file)
Speaking of free software I’d tried Handbrake, and i remember it not being suitable in the past. However, I just used it before writing this for the same audio drift kind of problem we are talking about, and it worked successfully. It’s not hard at all , you just make sure to select the box that says Constant framerate, chose a destination for the file to output to and press convert (or whatever it says)
Maybe it’s what you meant when you said “Ensured a constant framerate” and I don’t recall why I abandoned converting the file with Handbrake in the past, but this time it worked like a charm, and so I that’s my recommendation. Simple, free, easy and fairly fast. And as you might expect, such a relief 🙂