I have/had a G.Skill 4000 C15 kit and a CH8 Impact sat waiting for a 5950x. The 3900XT has done me proud with MT disabled despite maxing at 1866IF and having its good cores scattered amongst the clusters. I was planning to do the same thing with a 5950x, push for keeping the same timings as my current system at potentially a couple hundred mhz more. After that it was fingers crossed for some decent cores on the same ccx and for per core voltage adjustments to finally appear.
Board wise its actually a pretty poor choice if you're going hardcore...
The VRM on the Strix B550i put me off, they went cheap on the mosfets and have since removed the info off the product page on their website (50A SiC639 and not SPS, same as used on some Asrock $100 A320 itx board ffs). Its a cheap ass board with Asus tax on top. Fine pushing a 5800x but not suitable for playing with dual cluster cpus imo.
Asus' Strix X570i is missing vTT range and bios flashback, instant avoid there.
MSI's B550i could be good as its 8 phase 60amp, but their lack of high XMP support puts me off (although it looks like a B450i redesign and I have two of those as they're so good at mem oc) plus I have no idea of the spec of those 60 amp VRMs other than they aren't smart stages. Also the single SOC stage above the cpu socket with no heatsink cover triggers me hard...
Gigabyte's B550i is only a 6 phase and its 90 amp stages aren't really an upgrade over the 70 amp stages in the Gigabyte X570i I already own (which needs fans to stay below 90c VRM temp with the 3900xt in prime at safe volts and doesn't mem oc or score quite as well as the MSI B450i).
Asrock is Asrock...just no.
The only board that ticks all boxes is the CH8 Impact. Good vrm, good bios, good mem oc, bios flashback, postcode reader. You can unplug the fancy soundboard if you dont need it (I use a USB dac). The £350+ price put me off, but its got a £50 rebate at the moment which swung it for me (
https://promotion.asus.com/en/uk/q4gamingweeks# ). Having the board in hand, its 100% worth £300, but if I'm not going 16 core I just don't need it.
I was ready to flip a coin between the MSI and Gigabyte B550i boards before diving on the Impact, I couldn't pick between the two. If you're considering the Strix B550i though the MSI has a clearly better VRM, I wouldn't run anything with 2 clusters hard on that thing. If still limited to 1900IF and 1:1 mem then it seems to me the MSI is best choice (assuming no mem oc issues - my guess is they just optimised 1:1 and as a result cant go 5ghz+ XMP) with the Gigabyte B550i a close second. Neither will be capable of running a 5950x or 5900xt hard without getting uncomfortably hot unless you add vrm fans or open bench, but both should manage them just fine in daily software.
Pretty much everything above is opinion based with a sprinkling of experience thrown in...