I feel that with current star systems defences are too easy to overcome.

It becomes better with the Era 2 - 3 defence improvements but it would be better if defending manpower would be based on population number.

Capturing a home system of your enemy is a simple one turn task.

For example 50 manpower for each population.

A system that has 10 population would have 500 manpower for defence.

This way capturing a large system would be more difficult than a small one (which would make sense).

Also the "rebellion" defence option should be able to gather forces bases on population (and lose more population if they die).

It is strange that a 2 population system can mobilize 50% of population while a 20 population system can mobilize only 5%.

Maybe you could choose up to 25% (or any other number, maybe based on political views) population to go to ressistance or something like that.