I want to set up a single large bottle bank at 4500 run through a reducer that I can set for the pressure depending on which tanks I'm filling.
I do dive nitrox sometimes
To kick off dont fall into the trap of thinking your a pretend fire station or dive shop if your just filling for private use at home.
These fire house guys are "sold" a 'BRAND" system from a salesmen on commision, hungry salesmen who's best interests are met by a big expensive compressor and with a continued expensive service agreement for parts and labour. Besides jacking the bank up to 4500psi each day every day adds additional wear and tear on your compressor that you dont need and guess what they are now at 6000psi which is why your getting ex fire house pumps cheap.
Your first consideration IMHO is do you really need 4500 psi in the storage tank/s.
Dependent on the storage tank volume V1 and the storage tank pressure P1 and the number of cylinders in the bank
You can simply use the bank as a bulk dump filler into your scuba cylinder V2 and
use the compressor to boost from the decanted residual dumped pressure up to the required final pressure P2.
Then the compressor can continue and fill back up the storage tank to this proposed lower pressure.
You tell us the size of your scuba cylinder/s V2 and the working pressure/s P2 and the number of cylinders in yur proposed bank and we can do the maths.
But for a standard 80 scuba cylinder at 3000psi (11 litres by volume water capacity at 207 barg pressure )
And assuming storage bank cylinder of 50 litres volume at 2500 psi (140 barg) pressure.
The choice you have with a single cylinder 2500 psi bank is either or:
But first off do you want to be jamming all day into the thing trying to get 4500 psi so you can make a fill in a 3 minues flat charging rate like a fire house does or would you be content to dump say from 2500 psi to 2000 psi into your scuba cylinder from the bank and then wait 4.75 minutes while the compressor tops it off to make the 3000psi both options make the same fill. Your call.