I'll leave here a suggestion that can solve the problem on all networks, however it's up to Khaled to make decisions.
In the Favorites Manager dialog, two options could be added.
1st - Join only after identification of the nickname
2nd - Add a personalization of a timer that mIRC would have to join the channels, so the user could set a time of 10, 20, 30secs after the connection.
I don't know if this suggestion will be easier for Khaled to solve this problem, because as he says and well this situation changes depending on the network and so the best thing is to create something that works well on all networks.
