I ended up buying Gladius against my better judgment, and got more or less what I feared I would: a WH40K reskin of Pandora. Combat is a bit more fun than Pandora's, but the game as a whole feels even more shallow and uninspired. The biggest issues for me are the almost complete the lack of a plot, campaign or any other form of organized gameplay. There's barely anything to do other than wiping out all other factions.

My only question now is: do any of the DLCs fix the aforementioned issues, or do they just unlock other races (which I assume are already present somewhere in the game files, since all DLC installers weight only 6MB according to their store pages)?

I can't confirm 100% because I only got the faction-DLCs I'm interested in, but I'm pretty sure they are all the same. Meaning they only unlock the faction you paid for.
I'm going to say the gameplay for the DLC-factions tends to be more interesting than the 4 default factions.

The pricepoints are still insane.

Thank you. That's a shame, but I guess I'll just chalk it up as a loss and look for something else to play.

the dlc add what it says on the box... an extra race or one unit per race

yes this game is 'limited' but thats the point... warhammer 40K is about war and 'talking' your way out of trouble is NOT an option

Yup OP, welcome in a WH40K wargame.

Good thing that I forgot to complain about the lack of diplomacy then, mmm? ;) But no, that's not what I meant. I'm just surprised by how little content there is compared to any other WH40K game I've played so far. I suppose DoW has spoiled me, but still... at times it feels more like a demo or an early alpha build than a complete game.

As Gladius is by far my prefered W40K game (925 hours played), I'll look for those you prefer. What are those? (I mean, drm-free)

Just wait for the GOG release of DoW then, it's the only one I'd wholeheartedly recommend. :)

Rites of War was alright, the first dozen missions or so were quite enjoyable, but it kinda got tedious after the big plot twist in the middle of the campaign. I'd still rate it slightly better than Gladius though.

Still have to try Chaos Gate and Final Liberation, but I plan on doing that soon.

Sure! but I equally prefer old DoW1-SoulStorm and Gladius.