If that is a good question, we need to think carefully about how it is implemented.
It's not going to be perfect at first, I guess implementing and finding exploits.
What occurs to me is that a good assimilation needs to have peace after the conquest or obtaining.
Assimilation should not happen if the territory is not connected.
Therefore assimilation occurs over time, time is valuable.
But it can be stopped if someone tries to recapture or capture it, the question will be if a third party who is not the original owner or the conqueror tries to capture it, what would happen?
The situation is as follows, A has a tower, B captures it and begins to assimilate it after disconnecting it from player A's territory. But C arrives and tries to capture and is not an ally of A, it belongs to C team a third party.
This may have happened during the wars of the Diadochi.
The wars of Antigonus, Seleukus and the Ptolemy.
You have to think it over carefully.