Still sounds like muffler blocked. Not the main exhaust but spark arrestor way down in the tail muffler. Sorry if i've been here & said that. Old age has me repeating stuff lol.
Run it without the complete tail muffler & give it good rev out--if you can stand the noise. Be sure you can poke a bot of garden hose in past where the muffle pipe ends in there as can carbon over at that point. But ok you've burnt it all out so i'm not making sense--again. Video is good--held wide open for a listen.

Pulser coils play up on those although if faulty on advance it wouldn't get past a slow idle. If faulty on the first half then wouldn't start at all. Think that's how it sort of operates on the early ones.
I see you say it 'revs & idles ok until under load which would be good to hear as in theory if its the muffler as bad as i'm thinking then shouldn't rev when not under load--maybe rev up a bit on a light throttle but not a full song rev. I'm probably way off the mark so don't follow me too far down the rabbit hole.

I had this copied to past. Not sure why now but think i was looking to see if it had a high speed coil, but its just a lighting coil piggy-backed onto the ignition source for somewhere to put it.

www.partzilla.com/catalog/yamaha/motorcy...a/cdi-magneto-dt360a