Yeah, it's only EU that's ramping up. If heat output was variable, it would be near impossible to hit the equilibrium point without accidentally slipping over or under it.
I've had a similar idea for a potential reactor as you did, though I'm not sure if I'll actually build it anytime soon. As "cheap and easy" as these look, the MOX fuel itself is actually insanely expensive. For the 16 cells in that setup, you would have to run 432 normal uranium fuel rods through a reactor. Even with large reactors, that takes quite a long time. By the time you can actually build the cells, the cost of the reactor is probably entirely irrelevant (at minimum you're going to have tons of components from your other reactors that you could be reusing).
Thankfully the MOX rods, once you have them, will kind of recycle into themselves so long as you have enough U-238. That 2x2 quad MOX setup will consume roughly one hundred U-238 over one and a half hours.