I have a few ideas on how to improve ground combat.
Firstly, implement a basic division builder system. Nothing as deep as Hearts of Iron which is literally a game all about simulating world war two era combat but a system that lets you have some real thought and choice into constructing your armies. Secondly give some more stats, right now there is only health, damage, and morale health and damage. Some extra stats could go quite a ways into further increasing the meaningfulness of your design choices, much as how space warfare currently has somewhat meaningful choices in fleet composition and ship design.
You could have say, regiments that you can put into a division and "modifier cards" you can put on regiments within the division to modify them a bit further. Such as a power armor card you could put on infantry or animalistic (i.e xenomorph) regiments as well as other cards like genetic enhancements (though gene-warriors would remain a separate thing), cybernetic enhancements, psychic powers (the difference between an assault army with a psychic power card and a psi-warrior regiment would be like the difference between an initiate and a yuri clone in Red alert 2.)
Don't try making some mini-board game out of ground combat, that'd be silly and tedious and literally no Paradox Development Studios Game in the history of ever gave you that kind of control once a battle was joined. Not even Hearts of Iron and that's the dedicated wargame out of the lot. Instead just give some meaningful choice to be had with unit composition and design. This would require some extra stats to make it really meaningful; you could put in things like speed, frontage (basically how numerous a force is), range, defense etc. While these would ultimately just be assorted multipliers and additives like all stats in Paradox game combat ultimately are it does give at least the impression that more is happening.
What I think is crucial however, is giving some extra visual feedback. Ground combat basically has none as it stands. Not even the bare minimum of watching two gigantic dudes poking each other, My proposal is quite simple, just replace the current really visually uninteresting battle screen with something more akin to HOI IV's battle screen. Ultimately it amounts to the exact same things under the hood, it's just two piles of numbers crunching away until one pile of numbers is left standing, but it would give the illusion that something cool is happening.
I'd also recommend being able to build some special tile improvements and orbital stations to help make it more possible/plausible for a defender to hold out long enough for reinforcements to arrive. The planetary shield improvement could do with a pretty major buff, like making orbital bombardment flat out not work and thus forcing the attacker to have to make a ground assault against fully prepared fortifications a la the battle of Hoth. Secondly surface to orbit defense stations and orbitally mounted satellite guns that can further harass and annoy any attacking fleet like the Ion Cannon at Hoth or the Magnetically Accelerated Cannon Satellites around Earth in Halo. Even basic things like hangars for aerospace strike craft because you all know me and how much I fucking love space fighters even if they're the least realistic thing ever. You could also further raise the importance of sieges by giving the defender some added bonuses like new kinds of military stations including interstellar ranged artillery and interstellar ranged subspace snares that force hostile fleets to have to go through a particular system; thus allowing for "Fortress worlds" like Cadia and making for some actual game geography outside of Hyperlane games.
The other big thing I'd recommend is as I've repeatedly advocated, dealing with the Doomstack by decreasing the importance of decisive battle by making it much easier to come back from losing a fleet. This would favour more spread out forces that can take out an enemy's industrial centers and gradually strangle an enemy to death by eliminating their production powers rather than a big slow moving blob that tries to hunt around for that one battle to win the war. This would make sieges more important and more even.
This all adds up to a pretty thorough reworking of how warfare works but I think it would make combat much more involved, fun, and more "epic" to use a word that I'm sure the early 2010s and late 2000s made us all sick of hearing. This, along with my hopes for buildable titans and leviathans via megastructure shipyards and megastructure death star esque planet killers (also a buff to strike craft like much more range, much greater numbers of strike craft, replenishability, and faster strike craft that can be upgraded with FTL systems to let them instantly pop over to a target in the system as well as dedicated carrier ships; please paradox I need my carriers

) would in my eyes help elevate Stellaris to having one of the best combat systems in the Space strategy game genre.
Also unrelated but; I need my Mecha Paradox. Give me my big stompy walkers and mecha. I don't care if they're gundam esque mecha that can outfight spaceships in space, warhammer esque walking fortresses, or Battletech and Star Wars esque tanks with legs but I need my mecha. Hell you could go for all three and I'd be fine, I just want my big stompy death robots to go along with the trainable pet Kaiju already in the game. On a more related note, Stellaris also needs an army and a fleet template order system. You create a template of the kind of fleet and army you want; how much of that fleet or army you want, then select the planets/spaceports that will produce that force and they'll produce it automatically. This way I can just order A number of battleships 1, B number of battleships 2, C number of cruisers, D number of destroyers, and F number of corvettes rather than having to piss around with manually assembling a fleet. As well as a button to replenish the fleet or army according to that template if any are lost and for armies; a button to return each army to its planet of origin for garrison duty.