I never seen such a setup you describing (which doesn’t mean it doesn’t exist). I don’t think you will ever get a nice control using a relay. When banked injection is used the ecu should provide full injector control.
Unfortunately the SQ6 cannot handle 16 Injectors. The MQ12 can.
Nowadays Injectors can handle huge flow rate as well as a very precise fuel delivery with low duty cycles which makes banked injection more and more obsolete.
What specific power output you are aiming for per cylinder/injector?