Hmm.. Those Marvell controllers are problematic, mainly due to the drivers being so out-dated and they misbehave when it comes to signal/protocol standards.. Don't mean to sound like a d*ck here but a while ago i was doing what you were all doing, keeping that platform running as long as possible and kept spending little bits here and there to try and improve it. I never used NVMe drives on there because that would be a massive bottleneck for the drive and unfortunately you'll still get low frames and hitching! Once i went from a GTX 970 with my Xeon to a 1080 Ti, sure i could run Forza and many other games in 4K but i couldn't achieve 60FPS on many of them, often dipping to 40 even though the card was capable. As soon as i was using that same card on the older Z370/i7 8086K i saved up for (i'd purchased a USB3/Sata 6G card for my old X58 board, which did improve my SSD speeds but not by much - from memory it went from 254MB/s (about the limit of Sata 2/3G) to 345MB/s.. Of course on the newer board it hit its max of 540MB/s, a SanDisk 240GB SSD) i was getting over 80FPS on Forza Horizon 3 at 4K with 83% GPU usage, for example. On the Xeon it was 40-57FPS at about 50% usage, just couldn't pump the data to it! Depends what you're using it for really but that's not an overly demanding game either. And that was loading from the same HDDs, by the way. Didn't have to reformat either. The good news is, if you have RAID arrays on SATA drives, if you connect it to a newer Intel board, it gets detected and works immediately. I was surprised myself (i had a RAID of 2 2TB WD Blue HDDs for games at the time) and it literally just worked! Taken a few years to get the machine where i want it to be, but it can be done cheaper than you think! I do have a few X5650 chips here though, i think i paid £6 each so got 4 of them, intending to 'bin' them for a bit of fun but still haven't got 'round to it yet lol If they had ever been updated to use PCIe 3.0 that would likely help a lot!
This does make me want to dig out my X58 board and mess around again, but i really can't knock how well a now 11 year old CPU can still perform. An i7 8700K (which is what the 8086K is a binned version of) is quite an upgrade that would surprise you however, and perhaps when things return to normal, prices will actually be reasonable again! Running XP on the old X58 Xeon as someone else mentioned, that was hilarious though! Never had it feel so fast
And if you want stability, I'd say avoid older Ryzen platforms. I used to be the biggest AMD fan back in the day but once i realized that really, you're spending the same over time just in smaller amounts, i went with Intel (the X58!) and never looked back! I suppose the only annoyance is, besides an old IBM server i have that uses dual Xeons and has UEFI, only AMD's awful FX series of CPUs has UEFI from near that era..i think we can all agree they were trash. Eight core indeed..