Haven't played it yet, but yeah, from the descriptions, sounds like it could use some changes. Maybe:
-Built along the edge of the border.
-Does not take a builder charge
-Does not destroy features/resources on tile, and another improvement or district can be built on the tile. However, if there's an improvement on the tile, the wall section is only 1/2 as effective. Also, any wall section that's no longer on your border would only be 1/2 as effective, and if you build an improvement on a wall tile that's not part of your "outside" border, the extra wall section would be cleared (so that if you want to develop your city, only the outer-most wall remains)
-Each turn you can only build along one border (so that each wall segment basically only acts as a defense from attack from one tile)
-Yield for it obviously would need to be adjusted to not be too OP.
-In this case, you could have an encampment on the same tile as a wall, and that should give a larger bonus
-Barbarians should not be able to cross a wall section
That way, yes, one builder could build the entire wall for your empire, but you're basically going to need to spend a couple turns per tile to build it, which might not be the best use of your builder's time.