Best course of action? Is to immediately flee to the nearest friendly retreat side of the battlefield and wait for your allies to show up and refuse engagement. This would be very easy to achieve.
Imho this is a detail that can be worked out.
The reinforcing armies can show up later, and can also have maluses attached to represent a forced march to relieve the attached forces.
Total War approach is not really 100% ideal for planetfall.
I never said or implied it was. I use it as an example because it works quite well.
There is zero risk in this, simply because there's no very long ranged shots in planetfall like 20+ range. And if such a thing shows up, then superior tactic would be to whittle down the defenders with long ranged shots while maintaining range.
Tactical operations have unlimited range, yet have enough limiters that they don't unbalance things.
Look, none of these ideas are perfect, but using them I think would help the problem highlighted in the OP.
If you attack an army that is in a 3 hex line, currently you only have to fight 2 of them. That's a bit abstract anyway. Under the proposed change, you would fight all 3 armies, but there'd be a delay on the 3rd one showing up, so you'd be considerably more careful, as a Human player, trying to split stack the AI.
And I don't think it'd make much difference to Human on Human conflict.
By keeping the economy as tight as it is now, you'd very rarely get very large armies (and if you did, they'd self balance to a degree by being all in one place.)
In total war, soldiers if they see friendly soldiers fleeing the battle, they get morale debuff and would want to flee. This would transfer to like -600 morale penalty to reinforcements who is trying to support the army stack that got raided in planetfall.
Planetfall doesn't have a morale mechanic like this, so this simply wouldn't happen.
By raid, i mean things like, going back to our army in a 3, or even 4 hex line, one of those stacks has a high value target, e.g. a leader or hero.
Currently, you can try and split stack that, and destroy it.
Under the new proposals, you can't "split" the stack, but there is a delay for reinforcing stacks.
So you can still try and assassinate that high value target, but the tension is higher.
It's worth mentioning that the 2 ideas I highlighted earlier were originally suggested in the context of redesigning AoW3, and also included things like army supply, leadership value and logistics, all as soft counters to massing huge armies.