Some general thoughts here:
That motherboard is a poor choice. Why? Several reasons. First, it's DTX, which drastically limits case compatibility (look around the Meshlicious and Dan C4 threads for
@Boil's compatibility woes if you're in doubt). Second, it has its m.2 slots on a separate "SODIMM.2" AIC, which further limits compatibility as it sticks out quite a bit from the board, making it incompatible with many sandwich cases. What do you get in return for this? Nothing much. Unless you are an LN2 overclocker, it has pretty much zero useful features over the X570 Strix, and arguably B550 is a better choice for ITX due to its much less power hungry chipset (which is why all X570 boards have fans), and the fact that you're not actually utilizing the PCIe 4.0 capabilities of the chipset in ITX. Yes, you get two 4.0 m.2 slots rather than one 4.0 and one 3.0, but ... the difference is negligible. You'll see it in benchmarks, but
even the fastest PCIe 4.0 drives don't come close to saturating a 3.0 bus in real-world workloads (look at the ATSB Light results for a representative average end user load, or ATSB Heavy for very heavy workstation use). A 4.0 drive in a 3.0 slot might perform marginally slower than this again, but nothing that would be even remotely noticeable. One could of course argue for a beefy VRM or things like that, but under non-exotic cooling you can't really overclock Ryzen very much, so VRM quality is plenty good enough on most motherboards. Most B550 boards also have
better VRMs than most X570 boards simply due to being newer, and they often also have advanced features like 2.5GbE that no X570 boards have. There are no good arguments for X570 being more future-proof than B550.
I would look at the ASRock Phantom Gaming B550 ITX/ax, Gigabyte B550I Aorus Pro AX, or Asus Strix B550. Each has pros and cons: The Strix has a good featureset and great Asus BIOS, but the weakest VRM of the bunch (by quite a bit). Not that that matters much in real life. It's also rather cramped, and expensive. The Aorus is good and affordable, but lacks front USB-C support without an adapter. The ASRock has right-angle SATA ports making it difficult if not impossible to use them in many cases with the PSU placed directly next to the motherboard. There's an MSI board as well, which should be decent, but I can't stand MSI as a company, so I'm not going into that.
That PSU is complete overkill. It's not overengineered - there are lower output units that are as good or better quality - it's just way more than you'll ever need. A 10900K + 3090 can run on a good quality 650W unit, though 750W is a safer bet due to power spikes etc. Most PSU calculators are utter garbage, and don't come close to real-world power draws, ending up recommending
way overblown PSUs. The aforementioned 10900K+3090 setup is likely to consume <500W on average while gaming, though of course this fluctuates. If you're likely to run heavy 100% CPU+GPU workloads like rendering you'll need to account for the full power draw of your components, but with an AMD CPU, that's 144W max (they don't go higher unles you OC, period) + however much your GPU consumes. And given that the 400W 3090 is already pushing the bounds of what can be powered and cooled in most PCs, there's no reason to plan for anything higher than that. Add another 75W for the motherboard, RAM, fans, pumps, RGB, etc., and you've got 620W
absolute maximum sustained power draw (of course most 3090s don't pull 400W average - that's the OC versions). Real-world power draws are likely to be much lower, and of coruse using a lower end GPU will cut that number significantly. The 750W Corsair SF Platinum is the de facto standard in this range due to it having been on the market for a while and performing excellently, but any other 750W-850W SFX or SFX-L PSU will be plenty good. Focus on reading reviews and looking at noise, fan profiles, etc, as that can end up mattering more than output capability in the end. There are no cheap or low quality high output SFX PSUs. There's no reason to think the PSU you've chosen is a bad choice, but it's just a waste overall.
As mentioned above, don't buy 64GB of RAM unless you actually need it. "Productivity stuff" is pretty vague, but "maybe a little editing of graphics and video too" makes me suspect it won't be very RAM-heavy (unless you do lots of database work or compiling?). I recently upgraded to 32GB, and ... I don't actually need it. I leave literally everything open all the time (recently it's been 7 Firefox windows with ~15 tabs each, a few Word docs, Outlook, Teams, OneNote, at least three game launchers, with HWinfo64 and Aquasuite running in the background. And I
barely exceed 16GB while gaming, and at worst push a bit past that in Lightroom. Ryzen responds very well to faster memory, so buying the capacity you need with the fastest timings and clock speeds you can afford is the best approach. If you're currently close to maxing out 32GB in your regular usage 64GB is reasonable, but if not, stick with 32. RAM needs grow very, very slowly.
The best AIOs out there are either Arctic or EK. They are a clear step above the competition in terms of performance, and should be pretty quiet too (especially the Arctic).
I completely agree with your overall mission of making this a future-proof build (to the degree that such a thing is possible with a PC), but making smart choices is crucial, and going overboard on things where you don't get any benefit but just added expense or complexity doesn't help this mission.
But the main thing is the massive price premium for the C8I, which it really doesn't even try to make up for in features. It's no doubt a great motherboard, but there are great motherboards at half the price too.
