I think the focus on increasing the cost and benefits of planet invasion partially misses the point. Planets, like fleets, represent a limited number of of targets. The "weight" of the targets is not the issue but rather the entire concept of target-focused conflict. It restricts strategic choice to deciding on a target and attacking/defending it at the right time with the right force. Such a target-focused war dependent on fast reaction does not fit well with a strategic game.
What needs to happen is a frontline-focused war where progress is gradual and dependent on continuous decisions on part of the player, as opposed to momentary decisions. Meaning you can easily increase and reduce the forces allocated to a front and no single well-timed decision can win or lose it.
1. The hyperlane network is a natural frontline many space games use to allow players to entrench in nodes and force a slow war. Stellaris already has that to some degree in hyperlane-only games.
2. A system could be an abstracted frontline by introducing system control - a measure of how much each side controls warp entry points, has superior recon, minor hidden defences and mines, ect. However we need mechanics for figh
One such mechanic could be giving corvettes a usage as police/raiding forces. Basically they can enter a system largely undetected and covertly start establishing system control unless they're confronted by enemy corvettes serving as a police force. (the combat however is a slow-burn shadow war, as opposed to a quick showdown) The presence of fleets can also have an effect but not as much as these smaller and faster corvettes working in the system fringes. Essentially this system control would abstract how much each side can disrupt enemy supply and trade.
The effect, aside from economic disruption, would be that system control gives major advantages in fleet combat (due to better recon, supply, ect.) and ground invasions. This system can also be used to introduce pirates, privateers, ect. to encourage using police forces in peacetime.
3. A planet could be a frontline if invasions happen in several stages, with both sides having time and possibilities for reinforcing and evacuating forces and allowing for temporary stalemates if the defenders can't push the invaders out of their beachhead but the invaders also can't assault the defender's fortified positions.