1. Do you have numbers on power usage? Your spinning rust drives are using 5-10 watts each. It’s likely that they’re using more individually than both Pi’s combined.
2. This is fixable with more software
3. Unfortunately a part of life with Raspberry Pi. Maybe consider swapping to an SSD?
4. This is a 12 year old Synology. I empathize though, I’m running a DS416j I bought in 2015, so almost as old.

My suggestion would be to focus on getting a better Synology with an Intel chip that can run Docker. You’ll start off being familiar with the base OS and can make your backups rock solid and you’ll be able to run whatever you want with Docker. Assuming you can swing a 4 bay NAS you’ll also have room for expansion.

I would take the door off and put the rack in front, if you can make it work. I would also get a rack with caster wheels to make it easy to move it if I needed to do something in the cabinet.