Glad you found it, the quality of the Panasonic 420 and 820 players is excellent, I have used both and got my in laws the 420 when I got them a 55" Panny OLED from the "demo surplus" for just CDN$600 Panasonic employee cost. Unfortunately they have issues with MKV, not only cannot read Atmos or TrueHD but if you rip and backup a disc that has both Atmos/TrueHD and dts-MA like Creed, you cannot change audio tracks on MKV files so you're stuck on the "audio format not supported" track. If you're not planning on backing up your discs and using MKV, and don't care about SACD/DVD-A, then it's a smashing option.
I stuck with the X800 original model (Sony) as I needed to replace my BDP-62FD which crapped out and was going to be $150 to repair, got the Sony for US$228 in late 2018. It too has MKV issues with Atmos/TrueHD and Atmos/DD+ but at least you can toggle the audio options. It also has some issues with M2TS files where it cannot pick up the running time/elapsed/remaining and therefore you cannot FF or RW, Avengers Endgame is one example...I was starting to back up to M2TS just so Atmos would work.
Anyway I got an Nvidia Shield two months ago and that solved all my MKV problems. Not only that but the odd M2TS file I still have can be cued and will pick up where you left off which the Sony didn't do, no issues with Atmos/DD+ or Atmos/TrueHD. Fantastic interface for Plex, really impressed.
One caveat with the Shield, it will only bitstream what your receiver can handshake with...so I cannot use Plex/Shield to stream my multichannel FLACs and converting to WAV makes no difference either. I still have to use the Plex/Sony for those and that's okay as it works like a charm. If your receiver will not play MCH FLACs from USB, it won't play them using a media server with the Shield.
Congrats on the Panny pick-up, I'm sure you will love the 4K picture especially with an OLED. I went from a 60" VT60 Panny plasma in late 2018 to a 65" 4K Panny OLED, got it ISF calibrated and have never looked back. Stunning. Shock and awe.