Yes polish the chambers, cut the seats, then port.
Not even joking here, i use a harbor freight grinder and champion burs that I buy long and then cut the shanks down for what depth i need. I also have a speed control...but i just keep thumbing on and off with the switch.
I dont use cartridges on sand paper, I do it by hand with 80 grit and mainly on the exhaust side and to just check for surface consistency....otherwise i just finish with the bur.
Control the grinder, it's not gonna be nice if the bur bites digs in...and i have a couple L shaped shanks to prove