Haven't read the whole thread, but in my experience, you don't need to obsess over finding the best SSD for a Windows 9x or XP system. I've used a dozen different CF cards in DOS, 9x and XP, I've used repurposed 16GB Kingston M. 2 SSDs ripped from Chromebooks (using three adapters to connect to IDE) in 9x and XP, I've used old 64GB Crucial C300 SSDs in 9x and XP and i installed a Samsung 830 128GB in my wife's Vista laptop in 2012 which she used... Running Vista, which doesn't support TRIM... For about 5 years before I got her to upgrade to Windows 10. These are all MLC (and used for years), and I've never seen an SSD based system bogged down at all, let alone to the point that it was worse than a hard drive.
You can literally use almost any storage medium from the past 10 years and it will run Windows faster than nearly any hard drive made before 2006. If you leave some extra space for overprovisioning, you'll likely greatly reduce long term issues caused by a lack of trim unless you're doing something unusual that really exaggerates write amplification on the drive. I've never had to do this myself, but I believe if you ever managed to bog a drive down, you can just trim the drive in a newer OS once in a while (either by booting a different OS on the machine or putting the drive in a newer system). TRIM is nearly instantaneous, so this is far less troublesome than having to defragment a hard drive... as long as it doesn't nuke your file system... someone correct me if this is not a good thing to recommend, as I've never had to do this myself. 🤣
Edited for clarification: Also, from my understanding, modern SSDs have better garbage collection features built in to the controller and can maintain themselves well enough to function acceptably without TRIM in light workloads... certainly those expected in a DOS\9x machine would qualify as "light" by modern standards. I could be wrong, but I don't believe CF cards have any of these features, and yet people use them all the time for retro systems with very few problems. I just found a nice simple article about TRIM as well. If it was recommended but not 100% necessary 5 years ago, then with newer drives on an OS from the 90s, I don't think you absolutely need it.
It's also worth noting, my 98SE tester system has been running on a 16GB SSD like this for 2-3 years. The way I test hardware, I actually make copies of the Windows folder for each configuration (for example, one for testing Voodoo 3 cards, one for testing TNT cards, one for testing Sound Blaster ISA cards), so I have it boot to command prompt each time and then rename the appropriate folder "Windows" to test a certain piece of hardware. Works great, but requires a fair amount of space and lots of copying to create new folders (renaming one to change the configuration is instant however). I've never had issues with this, running on a super basic 16GB MLC drive that was used in a Chromebook for who-knows-how-long. No overprovisioning, lots of writes, no TRIM... and I use it frequently. It might benchmark more slowly now if I hooked it up to a new PC, but there's no way the difference is noticeable on the 440BX + PIII 850Mhz system in which it's being used.
Time Machine = FIC PA-2013 2.1 - K6-2 500 - 256MB PC-100 - TNT2 Pro 16MB AGP - Labway Yamaha YMF719-E - Midiman MM401