That's mostly correct. Both are possible and happen currently.
I'm not sure how having planets eat armies will do anything to stop it, though, which is the major point I'm arguing against.
A more permanent solution is probably to have a fleet power cap for each fleet. Instead of allowing 100k doomstacks, cap it. You can only have fleets of a certain size before they become "uncontrollable" and start getting detrimental modifiers to their combat scores. -5% damage, -10% damage, etc. successively applied to each tier or percent it's over the max limit for that fleet. In DH, you can only have Corps of sizes up to four divisions (HQs modify this a bit) before they start getting huge ass negative modifiers that kill their ability to attack.
Of course, you still doomstack, but now you have to coordinate a doomstack over 20k blobs instead of a single 100k blob. It's the only way off the top of my head that would improve the situation. Getting rid of armies is just nixing a tedious and boring part of the game that adds literally zero value.
You make a lot of sense. There should be a cap, or even better: a supply system. Only X amount of ships can be effective in the same system before their supply lines become overwhelmed and my oversized armada fights at only a fraction of it's strength. It would make war a lot more realistic and I'd have a strong incentive to stop blobbing my fleets together as much.