Yes, I measure temperature at a few locations with a probe thermometer, and at the point I'm moving the goods past, to make sure I've reached an even temperature, and keep well insulated during rests. I also don't honestly worry about a tiny bit of variation across the mash as long as it's all within a degree or so, just as I don't worry about losing a degree during a rest (and that's in fact the most I do lose.) It just isn't significant in terms of having any effect on enzymes. This does rely on good insulation.
If the OP would be making a bigger commitment to new equipment by going with a keggle instead of a rims, assuming that the cooler and some kind of boil kettle already exist, that might argue for the rims. OTOH I also have a cooler with false bottom, which serves as a lauter tun, while the same kettle serves for mash and boil. I will say that I think even temperatures and avoiding scorching would be more of a challenge if the direct fired mash tun also had the equipment in the bottom to allow it to serve as a lauter tun as well, since stirring the liquid in the dead space would be very difficult if not impossible.
There are many ways to accomplish step mashing, and the choice is always a matter of preference, priorities, and some degree of compromise.
