If I was to abandon Marvell drivers, MSU and just use the controllers with Windows Raid, would the data survive a fresh reset if needed? Would the usage risk be about the same, i.e. a bad save being backed up?
If you perform a reset of Windows, it normally only affects the OS volume. All other volumes remain unaffected. To remove other data, you have to select the option to "remove everything" and "all drives". If you did that, the software RAID configuration should still be retained, since the reset process just deletes data not partitioning and other low-level stuff, so when you reinstalled Windows it would immediately see the software RAID array with whatever partitions you had on it, but they'd be empty. You could also just unplug the RAID drives while you reset Windows, so that they'd be certain to be left alone, then plug them back in after the reset.
If Windows 11 does turn out to be a better option, I could trash the entire Marvell system and go with a 8 port SATA 3 card configured to 4 arrays using Windows 11. I would put the drives on the mother board (Asus Rampage VI Omega) but it only has 6 SATA ports.
You don't even necessarily need to change your hardware. The Marvell controller (Startech is the card brand) seems to be operating as a standard AHCI SATA controller right now, so you can just use that card and manage the drives through Disk Management. If you're able to delete the RAID configuration through DM right now and recreate it, you're good to go already. Like I said, DM is not the most friendly interface for managing RAID. There's no hierarchical view showing which drives belong to which array or which controller card they're attached to (meaning you have to figure out which disk numbers are connected to Card A and which are on Card B, if you didn't want to split an array across cards), etc., and no monitoring/notifications if a fault is detected with a drive or array so you can investigate and either fix it or determine that you'll have to replace a drive (which would require recreating the array and restoring from backup, since it's RAID0). A fakeRAID controller that has Windows management software like the MSU would provide those features.
Aside from driver compatibility (which doesn't seem to be an issue in terms of the built-in drivers Windows is using), one reason to go ahead and replace the cards would be for the interface speed. The cards you have are PCIe 2.0 x1, which means the bandwidth limit is 500MBps. Just two really good mechanical drives in RAID0 could potentially max that out while reading from the outer tracks, though it would depend on reading a large amount of sequential data to notice it. But 4 drives in RAID0 might sometimes be noticeably bottlenecked, both reading and writing. A PCIe3 x1 card would have enough bandwidth for 4 drives. If you have an 8-port card, you'd want to be sure it and the slot you use are PCIe4 x1 to be certain there is never a bottleneck, or get a card with more PCIe3 or PCIe2 lanes for equivalent bandwidth, if your motherboard has a slot that will work. (If it's a really old board, the CPU to chipset bandwidth might itself be a bottleneck.)
Also don't forget, Windows doesn't care what ports and controllers the drives are on. You could plug 6 drives into the motherboard and 2 into an add-in card and Windows will RAID them in any combination you want (even all 8 in a single array, which risks a lot of data being lost if a drive fails). The only potential downside is differences in performance of the controllers, since the motherboard's SATA could be slightly faster or slower than an add-in card in terms of latency, but I really doubt that would actually make a difference with this setup. You could use all the motherboard ports and then plug 1 drive into each of the current cards and not have any bottleneck issues that way.
For the MSU login, you could try your Microsoft account email address and password if that's the account you use to log into Windows, or try the shortened local username (your user profile folder name), or MicrosoftAccount@emailaddress. It's also possible that if Windows Hello is being used, using the account and password for any authentication might be disabled. Even though the MSU is intended for Windows Server, authentication should still function, but being tied to a Microsoft Account always complicates things. I don't really think the data loss had anything to do with compatibility with Windows 11, though, as Windows wouldn't have just deleted data from a functioning array (as you seem to have now) OR it just wouldn't have been able to see the array (as you found when the Marvell drivers are installed).
A LOT of that is user induced.
My current Win 11 Pro is 3.5 yrs old, still on the original install.
Yeah, Windows 7 and up became MUCH better about not building up cruft simply due to the OS. Windows XP needed a wipe and reinstall yearly, or more often, just because of its own updates plus driver and software updates. (Vista could have been AMAZING about the situation but nobody would care because it otherwise sucked so much.) I've migrated my current Windows 10 install across at least 3 hardware builds, including a move from an Intel system to AMD, just by moving the OS drive or cloning it to a new one. systeminfo shows the installation was 7/4/2020. I think that might have been on an HDD even. There may be some slight optimization possible by doing a complete reinstall, but nothing I'd notice in day to day usage. Some apps are absolutely terrible about just piling up update files, like antivirus.
Microsoft DID start using a delta update technology that will require Windows to retain update files indefinitely, because every later update is just the changed data from the previous major update, so it has to have all the previous ones to build the current update, like differential backups. That will definitely result in more storage space being used, as you can't "cleanup" update files, but not quite the same as cruft.