The cabling in your wall is, for all intents and purposes, never going to be messed with again and lives in a place that isn’t accessible. Ignoring any fire or heat related considerations, aluminum and copper expand and contract at different rates. Eventually that cable will fail as the heat from the extra power for PoE will cause deterioration. You’ll just replace the patch cable at the end when that happens, but the main run is a much bigger deal to replace. Don’t save a dollar now to spend it many times over later.

You need to reset and reconfigure the extender. Using the same password and SSID isn’t enough since there’s a unique ID in play as well, on the router end. That changed.

I’ve been down this path before. I made my old DS3615xs be my plex server and do 4k transcoding after upgrading the CPU. I now run it on an i7 NUC with hardware transcoding capabilities. Decoupling the plex server from the storage allowed for independent upgrades, and I could switch over to my backup NAS when I had to do prolonged maintenance on the primary. It was cheaper overall too.

The PCIe spec says you should be anyhow

It’ll only run as if it was in a 4x PCIe 2.0 slot, so 2000MB/s.