Looks like you have had some good advice while I have been out working
Difficult to know what to suggest because if you do upgrade a part of the machine then you find the next weakest link so you may only achieve a very slight increase in performance if any at all
I certainly wouldn't look at changing your stepper motors with your current set up, your current stepper motors are running way under what they are capable of just now
Stepping up to 24V will obviously give you an increase in performance but it might not be quite what you are hoping for, really depends where you see yourself going with this & how big a budget you have
Once you start to upgrade it can be a real drain on your finances lol
Adrian & ger21 have pretty much summed it up, would have thought your board was really designed more for 3D printers than CNC routers if your running 12v dc & max is 24v.
Like ger21 I am running Leadshine AM882 drivers with a 70v power supply, motion control is via Ethernet rather than USB, Gecko that Paul has mentioned seem to have a good name but what put me off the 540 was that I wanted individual drivers rather than one board for all 3 axis on my machine, that's just personal choice though. Gecko drives were also not as easy to come by in the UK when I rebuilt my machine.