And that's fair, you could stick with the OG concept of a population free improvement that has to be spread around. I do think my tweak to make it where its the pop growth itself that triggers the civilian unit removes a lot of the problems I noted above (removing the need for food as one big example).The objective of this was to try to incorporate all that extra land they get from that UA for something other than just defense. Trying to create some economic benefit for tiles outside working distance so that Shoshone are incentivized to maximize their tile claims, settle wider and further apart.
The biggest issue with the concept then is the late game scaling. I am still "growing" at size 4 or 6 or whatever, but I now have modern food bonusese, so I would expect the civilian unit production to skyrocket. You would probably need some secondary value to use those units, or shut off teh mechanic somehow.