On my bike I have SRAM Rival levers paired to TRP Spyre C mechanical disc brake calipers. I'd like to tune them to "brake harder"; currently, I have to press more than half of the way on order to start braking. I can't find any way to do that. I could change that directly on the cable on the brake, but there ought to be a better way.
I could change that directly on the cable on the brake
I think by that you mean undoing the cable attachment bolt, pulling the cable tight and re-attaching it. You are correct, that's not the way to adjust mechanical disc brakes, unless you have lots of cable slack that needs to be taken out.
Different models of mechanical disc brakes have different adjustment controls, so you should look up the manual for what you have on your bike. The Kona Rove ST has TRP Spyre C calipers, the manual for which is here.
The manual says there is a barrel adjuster on the caliper and a pad adjuster. (I think this moves just the outer pad so you would need to re-align the caliper if you use this adjuster, but you probably need to do that anyway).