Okay, forget the M die then.
Just remember, not all phases are created equal. There are some 8-phase boards out there (like ones with 60-70a phases) that are stronger than some 10-phase boards (like the Gigabyte X470 board that has 10 phase primary with 40a per phase).
Gigabyte Aorus Master looks amazing, if it's in your price range. I haven't seen the full lineup of MSI prices but I think the Master is supposed to be less than C8H, somewhere around $320-$340.
I am usually like that, but I will make exceptions for the right board. Last time I went smaller than full ATX was an old Abit board (NF m2 nView) that wound up being a killer OC platform. Still, if slots are a draw for you then you want to avoid smaller form factors.
Nobody will get max OC on air with a 3900x. People pushing for 4.7 GHz or higher static will need AiO or custom water. I'm calling it now. NH-d15 owners, feel free to prove me wrong.
If you're really not sure what you want to do vis-a-vis cooling, try the stock cooler that comes with the 3900x for awhile, get a feel for how hungry is the processor, and then go from there. You already own some Mugen 5 coolers so potentially you could test one of those if you like.
I appreciate your enthusiasm and helpfulness, but, right now, the whole phase things is ehhhhhh and I really do not think it is a big deal unless you really are going to max OC; the CPU will throttle and the system will shutdown before it matters if you have 40, 50, 60, 70 or a billion amp phases All that lends to more efficient switching, but, it is not the end all be all.
If I was really concerned, I would wait 6+ months for Buildzoid to run through a crap ton of boards, but, again, I am not worried about all that because I am not going for max OC or even any CPU OC right now.
I really have no problem running the 3900X on the cheapest 10 phase or 8 phase even! and clocking memory at 3466, but I want a few features beyond the basic X570.
I am fully prepared to spend $250 - $300 on a board, that is the reality of the situation, but I do not want to pay extra for fuggin wifi when I don't run it on the desktop (14 phase boards, most 12 phase boards).
Right now the MSI X570 Pro Carbon WIFI looks like the best all around package (despite WIFI) for what I want. And I am still dropping a chunk on TR3 later, I don't need a crazy board for Zen2 right now.
Cooling is what it is right now, and this is a 7nm chip, the same cooler works pretty darn good on the 2700X.
I am looking at beefy down draft coolers right now and will probably pick one up in a month or so.
I still have my NF7-S Rev 2 and mobile Athlon XP chillin' in the basement.
Sorry, but this whole endeavor of worrying about power delivery on things that have been planned and designed for and planned and designed for and planned and designed for is getting tiring.
(I ran / am running a Gigabyte B350 Gaming3 and R7 1700 with mild OC and have had no board issues, and that is a budget budget board, so, really, I am not worried about a mid range X570 right now )
I run good airflow and 80+ Gold power supplies, so, I have that covered too