Sounds very like the 50" Panasonic that I finally upgraded in late 2020. I'd had it professionally calibrated and it was excellent.
I replaced it with an LG OLED, which took quite a lot of adjusting to make it match the plasma in terms of brightness, contrast and colour accuracy - but it was well worth it.
Side by side, showing the same content, the plasma suddenly looked noisy and flickery. The OLED image is much cleaner.
The really big difference, though, is HDR. You really need to see it to understand why; it's not about having a brighter image overall, but about having an image in which specular highlights and other light sources actually look like lights, and aren't just white. I didn't realise that I'd never seen a display anything like it before; it really did show me something genuinely new, and it's not something I was expecting.
Oh, yeah, it's 4K too, but I struggle to tell the difference from 1080p on a 55" screen at a normal viewing distance. For movies I have a true 4K projector, and at that size, the extra resolution is both noticeable and worthwhile. Projected 3D with active shutter glasses is brilliant too, *much* more watchable than 3D TV.
Yes, HDR is something I'm keen to get into, I've always been a little bit of an avgeek.
For many years I had a 9th Gen Pioneer Kuro plasma, a fantastic TV, I eventually sold it as I got tired of the "dirty screen effect" and panel buzzing (unfortunately many Kuros suffer from this, and it gets worse over time.) I was able to adjust the settings in the service menu to the point that the image contrast was extremely good, I did get a tiny bit of maldischarge if pixels hadn't been used for 30 seconds or so, which showed I was really pushing the panel to its limit, but it looked superb. A great example is 'Gravity', where you get shots of the Shuttle against a pitch-black background. You just need to see it to experience it. By the way, if there are people on here that enjoy calibrating test equipment and so on, I dare you to dive into tweaking a Kuro PDP, as it has about 20 different parameters adjustable from the service menu, from power supply voltages to rise and fall rates to oscillation periods and panel resonance tuning. Truly a work of (engineering) art.
I bought it second-hand as well, though with a fault and fixed it (common for a regulator on the digital board to go bad.) I think it cost about £300 at the time, when working examples were still selling for about £2k. But, I kept it regardless.
Ironically, I bought the Panasonic fully working (I don't know how I got an ST50 for so little, it was the 2nd-from-the-top Panasonic at the time, but maybe "plasma" was becoming a dirty word.) It was one of the few TVs I ever actually purchased in working order, and it died about 18 months later. A small optocoupler on the sustain board had gone open circuit, causing the controller to think the HV was missing on the floating voltage side. Took about an hour to find and a scrap optocoupler from a Samsung plasma board sorted it out fine. Been fine for 4 years since.
I've been considering a late-generation OLED panel, and I thought I might as well go big or go home, so will get a 65" one. 4K still probably won't matter for that, but I suppose it'll make things look a tad better. Thing is, the Panasonic hasn't died, and it's got 20,000 hours on the panel now, with no perceptible burn in, so I'm in no rush.