Recommended caching server specs questions

I know its probably not the best but we are using 1 x Mac Mini for about 45,000 students, not here all at the same time for a University. but we can hit 15,000 devices connected to our network at any stage (still dont know how many of these are iOS but would think a large number)
Specs are:
2.8ghz i5
8GB 2TB Fusion
Single Ethernet

good thing is we have had so many happy staff / students with the increase in network speeds for updates etc.

We currently dont force updates to staff at any certain time which helps balance loads

Processor usage is always low
Highest network traffic in the past week is 22MB/s
memory usage is about 4GB

Total served is 15TB in about 3 months
cache is about 1.5TB

EDIT: just noticed you mentioned icloud storage but this should give you some idea, we do not cache icloud data

From what we've seen, disk speed and network throughout are the most important parts. I would definitely swap out for an SSD and possibly up the ram. The other (but more expensive) addition would be a thunderbolt to 10gb Ethernet adaptor (if the network will take it).

I haven't seen the CPU hit that hard though so probably wouldn't change that.

I'd buy a second caching server (or more), as they are peer aware. Also gives some redundancy.

I've used old dual SATA mini's for several hundred clients, but with a peer, & not had issues.