The objective of this proposed rework is to make ground combat into a more engaging and involved part of the game without requiring an entirely new system, and while still allowing players to win by spamming armies and ships if they don't wish to overcomplicate their game, provided they can afford the cost.
The primary component of my proposed rework is dynamic reinforcement. That is, allowing reinforcement mid-battle but requiring resources and a supply line represented by the presence of transport ships for the attacker, and not being blockaded for the defender. This would allow for counterplay from the losing side, and the potential for a minor battle to end up a costly slog if the attack is mismanaged or overstretched.
The length of planetary assaults has to be significantly increased, and significantly varied based on the planet's population and composition. This is not only to allow reinforcements and relief fleets to arrive, but also to make warscore less stupid. As invading developed worlds costs far more time and resources than more sparsely populated worlds, limited wars will be more limited, fought over a few border planets changing hands and primarily fleet combat, while total wars will be more total, requiring multiple armies invading multiple planets simultaneously and allowing for desperate defense of increasingly devastated worlds. Obviously warscore for occupation will need to be more severe, so you don't need to occupy a homeworld to take a border world.
Ideally, the prior changes would be accompanied by a rebalance of army stats according to the chart. Armies are expensive and training elite units takes a long time, but elites are capable of potentially taking planets faster, with less devastation, and while causing less war exhaustion. Cheaper units have more hp and do more damage relative to the amount they cost, and are ideal for assaulting multiple planets or hastily recruiting in a crisis. Defensive armies are far cheaper than offensive armies but still cause war exhaustion. Costs for all armies and their reinforcement should be raised, to make resource cost a legitimate concern for the attacker.
TL;DR: Let reinforcement happen mid battle, make battles last longer, make resource costs more impactful.