So everytime i try to join the server i get the normal the server is full message and the when i try to join again straight after or a few mins later it will freeze my minecraft and crash and i always need to restart it so if theres a solution to this it would be nice to know about and also it could be forge but im not sure

so as a side question just incase whats the recommended 1.16.5 forge for the dalek mod

Yes, I have encountered this bug as well, since the server is always full, this bug makes it so hard to join!