Please know that I am not trying to rustle your jimmies here.
Apple could be clearer, but when they say "one display up to a native resolutions of 6K (6144 x 3456) at 60Hz or 4K (3840 x 2160) at 144Hz over Thunderbolt or HDMI" they are listing included resolutions at certain refresh rates. 'Up to A resolution at X Hz, up to B resolution at Y Hz, or C resolution at Z Hz." You'll notice that for both displays, the paired resolutions go down and the refresh rates go up.
I would not read that and expect that I could run dual monitors at 5K 120Hz.
I'm also not entirely convinced that the Ascender user knows what they're doing. Or if they are getting dual 5K 120Hz (on a lesser mac mini), that it isn't some fluke.
I respect you for dumping a large amount of money into some gorgeous displays, and am just trying to say that if I were you, I would not expect to be able to do dual 5K 120Hz because Apple literally says that you can't (and any sales rep should have been able to tell you that).
I will say one thing, however; we all know what this stuff costs. I do not understand spending $6600 plus tax on displays and hooking them up to a mac mini that costs less than 1/4 of that and expecting everything to fire on all cylinders.
If you really want the dual 5K 120Hz, a mac that can provide that is within reach for you.
I hope this doesn't piss you off; again, I respect you investing in what you value, and I'd be disappointed as well, but I wouldn't trust some random internet user when they claim to do something that exceeds Apple's own support page.