Umm the Corsair 550w RMx is a seriously good psu and I don't see it in any way being incompatible with an ATX mobo. As regards the EPS, that's a real conundrum of a subject. It supplies supplemental 12v to the cpu, over and above what's given by the 20+4pin. For years you could slap a 4pin in an 8pin EPS and have no issues.
As is, the 3900x only pulls 150w or less, easily handled by a 4pin EPS, never mind an 8pin. An 8pin EPS connector is capable of 384w by itself, without help from the mains.
The extra pinouts on some mobo's are where things go nuts. There was a lot of supposition about actual power draws concerning the 3k series, so it's entirely possible the EPS headers are linked somehow, with some mobo designs that predate actual release. Or possibly it's just a X570 thing, since they all seem to be using 8+4/8+8 and it's not a power issue, but a thermal issue on the smaller amount of pins pulling higher amperage, so creating more heat per pin.
Yes, normally you'd not need extra EPS. The cpu won't be using that kind of power draw except in extreme cases, like LN2 super OC world record attempts etc. Even a good OC or hard usage isn't going to put that cpu over an 8pin+mains limit.
But maybe some boards don't see it like that and need it populated.
Newegg ships pretty fast. And you have credit. If all else fails, you have options for a larger psu, but I don't see the need. I pushed an i7-3770K at 4.9GHz (about 200+w OC) and a gtx970 @ 124%OC for the better part of 6 years on an Evga G2 550w, basically equitable to the 550w RMx, and have had 0 power issues.
I've not seen or heard any concrete evidence either way, from anywhere, what I do know is you can slap a 3900x in an X470 with a single 8pin EPS and not have compatability issues with such a good psu as the RMx.