Well, after almost two months on the new firmware, I'd say the problem is gone for me. I did everything I knew of that would cause one or more drives to fault including multiple scrubs, and I haven't had a single drive fault. Seagate sure took their sweet time fixing this problem though. I could have used this firmware two years ago and saved me many a headache.
Yeah, I can fully understand that! I think it's been difficult for them, I myself have been running some of these drives for 2 years already and never had an issue but it suddenly did surface in my new system. So maybe they too had issues tracking down the exact cause, still, although I'm willing to give them some leeway in that regard, they should have at least had something known about it, even if it was just a knowledge article stating "if you have this and this and such, you might encounter this issue, please call us". Would have saved them a lot of grace in my opinion.
Whatever the case, I think Synology blacklisting the drive (turning off write cache) really accelerated the issue and Seagate made sure a fix became available. Very sad something like that is needed to get this finally fixed. Because of my YouTube channel I have some media contacts with Seagate but even through those channels, no help was offered and I had to "fix" it myself, that's also why I started this topic here and made my article and video about it. That's been viewed over 3000 times now and I believe this forum thread even more, so hopefully this information is out there now.
If Seagate reads this, if you have a problem with your drives, don't hide it, be open about it, work on it, enlist us if you have to and fix it. The IronWolf series have been great drives for me but this does cast a dark shadow on buying advice if you handle problems like this.