If normal guardians aren't supposed to be similar to a cap of fleet ships in strength until they hit 4-6 of them, then shouldn't their spawn rate just go up some, instead of trying to redisign their mechanic?
Right now, it takes a bit too long for them to accumulate to those kinds of numbers.
And even then, there are only like 1 per guard post, that guard the guard post. This makes it a bit too easy to "snipe" one at a time. Maybe if they patrolled like starships do? Or if that would be too annoying, maybe 2 to 3 per guard post, but leave the planetary cap unchanged? (This would lead to the same overall guardian strength, but just with more noticeable "clumping")