Yeah the low level cap was a bit of a bummer, but not enough to ruin an otherwise fine game.
Chaos Rising is a worthy expansion if you liked the first one. It introduces the possibility of becoming Chaos space marines as well as 3 different possible endings. Pure Space Marine, Pure Chaos Space Marine and Mixed... Level cap only goes up another 10, and they removed the random defend missions. A few other minor changes, but it's definitely worth it.
I've just played the first couple of missions of Chaos Rising, and it certainly seems that the missions are more structured and adventure-like. Moving the level cap to 30 helps, but I don't think it's enough. I'd like to be able to eventually max out all the characters. You can't do that, so you feel that you missed out on something because you never get all the skills and powers. Accessory slots seem to be very tight, and having to choose only four teams can be frustrating, though it's probably simpler to handle just four groups.
It also seems a bit bizarre that they have this cover system that modifies your hit/defence, to the extent that you use it all the time for positioning, then as soon as you get Terminators or Dreadnaughts, it's almost impossible to use cover because they just smash everything they walk through. There's almost a very obvious line where cover is important, and then it becomes pointless as soon as the heavy team members arrive.
The inventory system seems a bit bonkers too. You level up some of your team, so you equip them with better gear and donate what you don't want to the library. That gives you more XP, so some of your other teams level up and you go and upgrade their gear too, get rid of their old stuff, and find that your first teams have levelled and so you need to go back and re-equip them again. I've probably spent just as much time in the load-out screens as playing the maps, but it is weirdly satisfying. It's probably why I was so annoyed by the level 20 cap that puts a stop to all that.
So it's not without it's faults and weird gameplay design choices, but you're right, it's generally quite a bit of fun to have your marines facing down hordes of enemies, crashing through the maps, setting up firezones from cover, finding better loot, and (my personal favourite) calling in orbital strikes.