I understand that you want your budgies to get along. A harmonious flock is definitely fun to watch and be a part of.
However, this is not possible in many cases. Budgies, like people, don't get on with everyone, and their needs must be respected.
In your case, you really shouldn't put them back together. Although I admire your efforts in "watching them 24/7", this is not possible as you will have to sleep and leave the room on occasion, as well as be engrossed in other activities even in the room.
It only takes a few seconds for an argument to turn fatal.
If you continue to risk the safety and well-being of your little ones in an attempt to help them "work things out", it is very likely it will end poorly.
Perhaps in the future they will be able to interact all together outside the cage during playtime, but they will never be able to be housed all together and shouldn't.
Also, I understand your plan about the cages. In the future, once they have had time to settle into their new routines, they may be able to interact under supervision, such as play in each other's cages while you're there like you are doing now. However, it's important to give them a break for a little bit so that they can regain confidence in their surroundings.
Housing two birds together and having one in the other cage is, in my opinion, the best option right now.
I know you want the best for your birds