But yeah it seems 80+ desynth is an insane stupid long grind that involves 20millions worth of stuff or running primals EX for 3weeks non stop, and not in DF cause too many wipes and long queues(unless tank, but still need to kill stuff).
Might vary between servers, but 20mil is quite the exaggeration. I'm not sure why you mentioned EX since nothing from them is desynthable, the primal stuff that you farm is hardmodes, which are braindead easy and easily farmable even in pugs. You can either get a group specifically for farming them to desynth or just queue repeatedly as healer or tank depending on desynth class.
Personally I got bored of trying that within a few runs, so I mostly just did PvP for fun/horsie and built up marks over time. I have only done like 50 FL matches and that was way more than enough to get pass the appropriate range level, (I'm actually 82 BSM or something now thanks mainly to PvP too). Mixed this in with artisan spectacles for the 70s. When the gains dropped hard I switched to 3star gold spectacles, if you make them yourself they are pretty cheap(as are a good chunk of 3star items now), and you can get 5% extra desynth chance for every materia slotted. The battlecraft III market is actually better than it was when I was doing this, and I was still making back a lot of what I spent. It's still a time/gil sink but it's really not as bad as you're making it out to be, and it is only getting easier for that first difficulty bump at i70 thanks to phil tome mats dropping. Coke was like 2-4k the other day.
The real question is if it is even worth it. The big repeatable/sane money maker at the end of the tunnel was Field3 and Mastercraft but they are all on their way down. with crazy fluctuations/undercut wars constantly, you gotta really get lucky or babysit to sell them lately. There is still some profit to be made there for now but it's getting less and less appealing. The primal demis have never moved that fast, are a pain to farm, and yet even they are dropping well below what they were. Ifrit is especially bad.
Maybe things will change if Supra are required for new 2.4 crafts, or if the next relic line really does involve demimateria like many have joked. In which case you got plenty of time to work your desynth up before then, utilizing the increasingly cheap mats.