I think the fact that Dave and Michael consider friendly suggestions from their dwindling fan base as ban worthy hate comments makes me highly doubtful they're very big on self deprecating humor.I don't think there is a consensus however the main suspects that I can see are:
PCDev and/or Bianca
They have feuded with The Trackers in the past. PCDev engaged a bit with the FRE comment section in the earlier days. The videos making fun of PCDev aren't anywhere near as bad as they could be - possibly made as an 'alibi'. Videos have dried up when PCDev has had other things on - e.g his recent cruise.
Shawn in Orlando
Regularly sounds off on Michael Kay/other bloggers on his Tiktok. I don't think this one is likely because there's supposedly a female editor involved with FRE. Possibly a contributor.
Michael/Dave themselves
Also seems unlikely. Unless this forum has completely misread Michael/Dave I don't think either of them are capable of the wit required to create FRE. I also don't think they would be self deprecating enough.
Craig/Ryno/Others from The Dis
Like with PCDev, the videos making fun of The Dis crew aren't as bad as they could be and seem to be the kind of self deprecating humour Craig would have. They certainly have the skills and resources to make it happen. Their channel has also been around for a very long time and the influx of these new vloggers must irk them somewhat. FRE also actively makes fun of Craig (self deprecating) but has stated numerous time that Craig's wife Kylie is amazing. The counter point to this is that Craig has seemingly got on well with The Trackers in the past, although this could have just been politeness.
WDWNT
I think these guys have escaped any kind of mockery from FRE despite being a pretty big target with Tom being widely disliked. That's really the only evidence.
Realistically it's probably none of these people and it's most likely a greasy 40 year old living in their Mom's basement lol