If it's not an RTS then I'm down to play it for sure. From what I've read online CK3 has a streamlined UI, without dumbing the series down. Since CK2 is free I'll try that as well. Do those CK2 DLC packs ever go on sale? $300 seems like a lot of money for DLC IMO.
It's real time (albeit pauseable) but it definitely wouldn't fit into the RTS genre. It's closer to Civilization or Total War (campaign map), although still very different from those, than it is to typical RTS games.
Whether it's dumbed down or not really depends on the perspective. It's kind of like The Sims, in that it's definitely not dumbed down (and is an improvement) over the base game of CK2. However CK2 + all DLC is still a long way ahead of CK3.
The CK2 DLC goes on sale very often, and you really don't need it all anyway. For starters you can completely ignore any of the music or content (they're cosmetics only) packs. And the others really depend on what you want to play. For example, if you have no interest in playing a nomad (eg. Mongol) then you wouldn't really even notice a difference between having their DLC pack and not having it.