I’m in the same situation as you are. I also have several regions filled with only rival tanks, all are lvl4 rivals.
Regions where i don’t have all 8 rivals per region as tanks, are: Archipelago (worst region to level up), Forest, North Coast (both regions are low level, <5) and Himfjäll (best region to level up).
So, i’m currently in Himfjäll, leveling it up and i hope that if the Reaper spawns, it doesn’t spawn next to any other of my 6, lvl4 rival apo tanks, that i have in Himfjäll.
I sure hope the current system gets a complete rework when it comes to spawning the reaper, since how it’s implemented currently feels counterintuitive to say the least - kill only the small critters to raise region score, which in turn makes that region spawn rivals. Kill a rival to “make room” for the reaper, and the region score drops again.
This is perhaps the most clumsy mechanic I’ve stumbled upon in any game, and it simply doesn’t make sense.
I have a question: Does the Reaper is bound to the same respawn timer as all regular Rivals?
I just leveled Farmlands to 25, destroyed lowest of the low Rival… and another regular Rival showed up 
That’s what I’m thinking as well.
Exactly the same situation last night, which spawned a Military Tank instead of the expected Reaper.
I was under the impression that with 7 active rivals, providing your region level is above 21 - it would spawn every time on the 8th rival. It would seem there is more to it
OK, I think that the Reaper spawns outside of the standard 2h timer. I just destroyed the fresh rival… along with 4 Tanks, 2 Harvesters, and gazillion of Hunters and Runners (NW part of farmlands 
) and finally it spawned.
Good man like it.
I’m streaming all my Reaper kills, so should be easy enough to figure out actual game time based on Reaper death to Reaper spawn based against stream clock counter.
A Reaper will only spawn at level 21 and above, The higher the Level from there the more chance you have of spawning Him (Keep in mind there is a 2 Hour gap between each Rival Spawn, I believe) The Maximum amount of Rivals Per Region is 8, and a Reaper Counts as a Rival, so you must make sure you only have 7 Normal Rivals in Your Region, in Order to Spawn a Reaper. Don’t Kill a Rival Unless you Absolutely Need to, Unless its the Reaper 