There are Mac minis on both end, one at home (which gets up to 900+ download) and one as the server (which can do 900+ uploads according to Speedtest).
As a first step, I’d get iperf (free command line utility that acts as both a server and client) and do some UDP speed tests between both sites in both directions to find out what the actual speeds are that you’ll get between them. Speed test apps/sites only tell you about the speed between your computer and the service, which is frequently only barely related to the speed between any two computers on the Internet.
If you think multiple streams will help you can test this by doing simultaneous transfers of two or more large files. If their aggregate transfer speeds don’t greatly exceed the single transfer speed that you see then multiple connections probably won’t help you.