Please Help Me Understand Downmixing

[volcanolotus]

I have a soundbar but I'm about ready to take the plunge and get a receiver. I'm trying to decide if I want to go 2.0, 2.1, 3.1 or 5.1, and I'm having a hard time figuring out what I'd actually get with different audio tracks at different configurations.

Setup is LG B7, Panasonic UB820, current model Chromecast (4K/HDR), and a laptop (less concerned about that for sound as it's pretty much only used for the kids to play games occasionally). I also used to have a laserdisc player and am strongly considering getting another. This is cinema-only (does not need to do double duty for music or anything like that). I lean more towards dialogue-heavy films (Criterion et al), but I assume a mono or stereo mix sounds essentially the same no matter which kind of system you're using. I just want the action movies to sound great when I do watch them

I think the easiest way to ask is to write out what I think happens in each scenario and hopefully people can confirm, or correct me where I'm wrong. (I'll also try to keep this post up to date, for posterity, since I haven't found this info in a single place anywhere I've looked.)

DVD/LD DTS/DD to 2.x/3.1: 5.1 mix with info on the missing channels missing (i.e., no downmixing)
DTS-MA/TrueHD (any) to 2.x: Downmixed to 2.0
DTS-MA/TrueHD 5.1 to 3.1: 5.1 mix with info on the missing channels missing (i.e., no downmixing)
DTS-MA/TrueHD 7.1 to 3.1: Downmixed to 5.1 with info on the missing channels missing
DTS-MA/TrueHD 7.1 to 5.1: Downmixed to 5.1
Atmos/DTS:X: Same behavior as 7.1

Further, I'm under the impression that 2.1 isn't really a thing and that the receiver just directs the appropriate frequencies to the sub from the 2.0 mix - is that correct?

Thanks in advance for any help here!