I found a used HP business small form factor i5 6th or 7th gen intel for very cheap, slapped a few SATA drives in the thing, and one of the M.2 Coral TPUs.
it is running 20-30% CPU load with 6 cameras on it - but they standard HD - I pumped one at 4k, and it loaded up much higher, so I scaled back to all 1080p or less. The TPU doesn’t even hit 1% from what i’ve seen. I should probably load a better TFLite model. Nothing mission critical - mostly a novelty.
not the most power efficient setup.
There are lots of them out there.
For example passmark is one of many.
https://www.cpubenchmark.net/cpu_list.php
I just go find my CPU and use the number to compare to eBay listings. Is it perfect? No.
But it gives you an idea. Each site has some set of algorithms and they get a score for how quickly it can execute on that hardware.
Some of them they allow users to run their system and submit numbers so you get a better sense.
I’ve tried a few of the things you mention over the years.
However, I’ve lately gotten into the used business PCs. The performance of even a 6th get Intel CPU more than double an RPI4 or the ATOM in my NAS, depending on how you count. Sure, it’s quite a bit more power, and they have their place (RPI in the garage), but I’ve gotten a few SFFs that have room for multiple HDs for like $50-$60 shipped, as long as i’m patient, since I don’t care for the windows license.
The CPU benchmark sites are what convinced me that more SBCs was not the solution for me.
I also tell myself that i’m recycling what could have been ewaste otherwise. I am afraid to calculate the energy cost.
For what it’s worth, Time Machine doesn’t seem to care as much which file system it goes to anymore, as long as it goes over SMB.
There are specific config settings to make it work , but nearly all servers , including synology, will have a button that does whatever the magic combo is.