I've seen testing.. done it myself. But look where you are now... didn't even hit DDR4 and on DDR3. So you'll never reap the meager benefits. Whereas if you get a 'reasonable' DDR5 kit now, when the platform matures you have the ability to jump up and get them. Whereas, with DDR4, you know your ceiling. DDR5 will likely be available on the next platform you buy as well. I totally get you though. In most cases it doesn't benefit. But if you can afford it without say dropping your video card down a notch, then I'd do it. If you don't care about that stuff, then DDR4 will be fine... but to move to DDR5 is a new mobo + plus the RAM.
Cases are more of a personal preference to me. I bought the Hyte 60 and love it. I would also stop worrying about a rad up front as the internals can handle warmed air from the AIO... like any case and any configuration, airFLOW is the key. I'd honestly worry much more about the carryover OS than I would with the rad in front and cooling.
You can through the apps I listed. You restore them/reassociate them through the same apps. When you go to install a game, you point it to a path/there are restore options depending on the service. If you check the service, you'll see the backup and restore options/methods.
That's how I do it. My 2TB NVMe is partitioned into ~250GB and ~1.75GB parts. My games sit on the larger part while the OS on the smaller partition. The most frequently used (and large) games are backed up to a HDD.