It seems to me, that there might still be the issue of not getting along enough well to share a cage. Since you say that even with free flight time out in the room (much larger area than any cage) they are still territorial about “personal space”.
Keep in mind too, that often times when adding more budgies and individual personalities, as well as hormones into the mix, it can escalate problems or potentially create new ones. If the girls are territorial with one another (or one of them is), then adding males into an enclosed space with them doesn’t sound like a good idea at all. Or if you are concerned about the one who has mellowed and slowed down, same thing.. it might not be the best idea.
Adding more birds (after quarantine), can add an element of stress not only to new birds, but to existing birds as well. Even when all get along, there is still the adjustment period under the best circumstances.
If they don’t get along, which may or may not be evident immediately, you’ll just have more birds to separate. Separating certain ones for out of cage time too, can be time consuming and big pain. I do this with my birds of various species who don’t all get along, and it can be a challenge.
Just some things to think about