I have a Synology NAS and it makes it all pretty easy. To replace/upgrade a drive you basically just take one out and put a new one in. Then just wait for it to finish and then move on to the next drive. Last time I upgraded my 4 drives took a couple days but was painless. Raid is great.

It really throws off the average though

Google Fi will throttle you after you hit a limit depending on your plan. I unknowingly hit mine after using my phone for a hotspot, watching a few hours of soccer and I think Windows downloaded a bunch of updates too. It was towards the beginning of the billing cycle so the rest of the month really sucked. Might want to double check your plan.

Sounds hard.