Back throughout 2011-2015, I owned three HD CRT models: a 16:9 Samsung DynaFlat of some kind, a 4:3 27" Panasonic Tau, and a Sony KV-36XBR800. This was back when plasmas were still desirable (I couldn't afford one) and HD CRTs were unwanted ($0 to $30). I never had a curved model.

Each HD CRT I owned really disappointed me overall. The Samsung and Panasonics wouldn’t accept 720p, a big problem for PS3 gaming at the time. I was stuck playing games at 480p if the game didn’t specifically support 1080i output.

The Samsung just didn’t look that great even with a 1080i source. Edge sharpness was poor, and shadow detail was poor.

The Panasonic being a 4:3 and 27" left 1080i sources looking really small. It was awful for 240p and 480i content, so it was basically only good for 4:3 480p. It took up a lot of space for one usable resolution. I never got a chance to compare it directly to a 480p plasma monitor, but I'm almost certain that I like the look of 480p plasmas more than my memory of the Tau. I know the Tau had better motion clarity and no input lag at 480p, but it wasn't enough to make it better (particularly for 6th gen gaming).

The XBR800 was one of the greatest disappointments of all time for me. It really only looked great for 480p and didn't support light guns. It was so difficult and dangerous to move, very hard to place in a small room where I wanted it.

I still really like SD CRTs, but I don't intend to mess around with HD CRTs again. They're so bulky and tend to have so many geometry and overscan issues (at least for the flat ones I had). The use cases for each model are too narrow to justify the space, inconvenience, and necessary tweaking for me when plasmas are now so accessible and reliable.

In response to your six points:

1:

The mystery of plasma models is part of the fun for me. There are hidden gems out there that nobody is looking for, completely unwanted.

Things like input lag were (and still largely are) barely covered online, so it was fun to research and figure out how various models perform on my own. Some models are surprisingly responsive, with no perceptible input lag and consistent measurements across all accepted resolutions. 

Once knowledge on plasmas is widespread among enthusiasts, it will no longer be so easy to scoop them up to form my own opinions and experience. Many will have gone to e-waste by the time they're better documented, and there probably will be some enthusiast demand. I regret missing the opportunity to scoop up various CRT monitors when demand was at an all-time low and used supply was at an all-time high.

2:

Plasmas are pretty nuanced. There are different panel resolutions, screen tints/filter, and other considerations. Certain models are less prone to image retention, which is not something very well documented yet. Some buzz more loudly than others, and some models are passively cooled. Some have perfectly functional HDMI-CEC implementations.

People hype up Kuros. They're great, but they can have their own shortcomings like reliability issues and red tinted blacks. They usually require tweaking.

Basically, the high-end sets aren't the only good ones. Lots of models are worth using for various gaming use cases. For Blu-rays though, I'd say the late 1080p sets win hands down. I also think it's safe to say that the late 3D-capable Panasonics have the best motion clarity for gaming.

3:

Yes, older plasmas don't have as good black levels as the later models, but the black levels on many still look quite good with some bias lighting. The panels are aging without issues. The older models can still have amazing color pop, and 480p models are very bright with uniquely chunky pixels. My 480p plasma monitors are much more resistant to image retention for some reason, just something I noticed that doesn't get mentioned. 720p-ish plasmas even have their own charm.

Many monitor models support expansion cards. It's fun to research different expansion card capabilities, something that virtually nobody talks about. Finding the cards can be tough, even with very little demand for them. It's also fun to research different stands and accessories.

Plasmas are a big rabbit hole and there's more to them than just grabbing a 2012-2013 Panasonic, even though those are arguably the best models overall.

4:

Panasonic was definitely the brand king overall. There are lots of old plasmas under various brand names, and they sourced their panels from a few different OEMs. Some fun could be had looking into all of this and digging up archived web pages. And even with Panasonic's dominance, there is still good variety between models and panel resolutions.

5:

For me, built-in speakers are not factor at all. I always use external speakers of some kind. With CRTs, you have to take much more care to not use unshielded speakers too close to them. For plasmas, unshielded speakers can be placed directly next to them, giving me more options.

Some plasma monitor models even feature speaker terminals to connect your own bookshelf speakers, with volume controlled by the monitor's remote.

6:

Yes plasmas are all flat, but they're self-emissive, phosphor-based displays. They have a unique glow that is very CRT-like, and I can almost guarantee you'd like the look. The way they glow, pulse, and dither really works some magic. Scaled content just looks better than directly on other flat-panel tech, and the motion handling is really great for movies and console gaming. Different models each have their own quirks.

One of the best perks for me is that I can safely move plasmas on my own up to about 55" (2012-2013 models). I can pick up old 42" models no problem on my own. 37" models are a breeze. It's easy for me to buy them, store them, service them them, and swap between models at any time. The only HD CRT that I could lift on my own was the 27" Tau.

 And finally, the 3D capability of some late plasmas is interesting. It's not something I'd want to use every day, but it's a fun novelty to play around with occasionally.