I made the jump from kayak tournaments to jon boats a couple years ago and think some of the experiences associated with that could help here. I ran a NuCanoe Pursuit with a spot-lock equipped XI3 that ran betwen 4.5-5 mph wide open. My current TM on the Jon boat is a Fortrex, so no Spot Lock. I can say for me, that I value the speed over the spot lock and it sounds like OP does too.
I am a power fisherman that likes to cover water, and once I embraced that, my performances improved. I used Course Lock far more frequently than I ever used Spot Lock as I was more effective hitting visible shoreline targets than trying to effectively fish offshore with a cheap graph. With foot steering, you could still steer and fish hands-free only needing to use one hand to adjust throttle occasionally.
There are times where I 100% miss Spot Lock, but not nearly as much as I though I would. I imagine you could hold position fairly well with manual adjustments of your stern mount motor the same way I hold with the Fortrex. Eventually I will upgrade to a TM with spot lock, but that's not a huge priority at the moment.
Like OP alluded to, most of the top anglers on the national level kayak trails seem to favor a stern mount over a bow mount for the ability to cover water. I still follow those trails to some degree and I routinely see folks talk about covering 10-20 miles a day in an event. That's just not really feasible with most bow mount rigs.
The biggest disadvantage I see with the stern mounts is that you have to spend most of your time seated in order to control speed and direction. I stood 95% of the time I was actively fishing in the kayak and think I would struggle to be as accurate and efficient sitting down. Maybe that's less of a concern to you.
Perhaps you should look at a faster hull design and more powerful bow mount. If I was getting 4.5-5 with a 55lb thrust in the Pursuit, I bet I could've rivaled stern mount speeds with an 80lb thrust and still kept the ability to spot lock.