An idea I was thinking about for a minute was a t4 planetary warfare ship. Battle Cruisers/Assault Carriers, either name works.
On average, the ship would have six large weapons which can be used for bombers, probably six large generals, and have the ability to train troops in space. Have a flagship level of 16.
The ship has access to a module which can train troops in space. Having this module, along with troop modules, the ship will gradually generate troops until the maximum number the fleet can hold. This would be slower than planetary bases, maybe half through 75% of the speed.
Using this ship to assault planets, you gain access to Dropships. Dropships would basically have more resistance to orbit defense systems, since they can maneuver and avoid enemy fire. Id say using dropships would bring a t3 orbit defense anti invasion down to 65%, instead of 75%.
What if invasions were activities instead of a set timer? In an invasion, the battle would be tracked on a 5 to 30 minute basis. Every time it ticks, you get updates for the progress of the invasion. Depending on the population number of the planet, your troops will have to capture say 2 to 8 bases on the enemy planet, you get updates as to if your troops capture each base. If you get an update saying they are failing, the next tick will result in a loss unless you react. You can launch an orbital strike against the planet, this preferably would be done by the Orbital Bomber and not the Large Orbital Strike Cannon. This strike would have near perfect accuracy since the troops are on the ground to give the target location. The strike would destroy the base instead of the troops capturing it, and the invasion continues.