The specs for this tv says it passes through Dolby Digital Plus, and actually processes Dolby Digital (probably for transcoding DD+ via Optical). TCL Specs
You could test that processing theory by setting the Roku TV for Dolby Digital (not Plus) and see if you get transcoded Netflix in standard DD. Normally, Netflix is only DD+ or Stereo.
Wait, when I go on Netflix it says it uses Dolby Digital. But I only keep the TV set to Dolby Digital, anyway, not Dolby Digital Plus. And when I use Netflix, it says Dolby Digital on the AVR screen.
I’ve also tried setting the TV to auto when connected via HDMI-ARC and it still shows Dolby Digital. I don’t think my AVR supports Plus.
Both of the AVRs you cited (Pioneer and Denon) absolutely support Dolby Digital Plus. They are advanced AVRs. They support up to DolbyTrueHD, a codec that's much more advanced than Dolby Digital Plus.
The only reason you would want standard Dolby Digital is if your soundbar(s)
don't support DD+. But the important thing to note is that your Roku TV is absolutely transcoding (converting) Dolby Digital Plus from sources like Netflix to standard Dolby Digital. That seems like it would necessarily inject some kind of additional delay into the output, since it has to do some converting before outputting.
That said, boxes like the Apple TV essentially convert all surround sound in the box and output it as multichannel PCM (unless you tell it otherwise). So, it's certainly possible to match audio/video when conversion is occurring, but I don't know if Apple is converting and then delaying video.
If your soundbar(s) support DD+ - and not all do - AND if you connect them via HDMI, I'd set the Roku TV for DD+. Otherwise, you're getting unnecessary processing on the TV side.
Note: you won't get DD+ over optical. Optical bandwidth is insufficient to carry the extra information in DD+ over standard DD, so if you're connecting via optical, you necessarily have to opt for the standard DD option.
Your Pioneer AVR supports "Auto" delay compensation, where both audio video delays are supposedly matched. It sounds like you've tried the "S.DLY" option for sound delay, but that only adds delay. Instead, try the "A.DLY" if you can. This should be one of the settings very close to the HDMI-Thru/HDMI-AMP setting. This is the blurb about it from your manual:
A.DLY (Auto Delay)g
This feature automatically corrects the audio-to-video delay between components connected with an HDMI cable. The audio delay time is set depending on the operational status of the display connected with an HDMI cable. The video delay time is automatically adjusted according to the audio delay time.
It may do nothing whatsoever for you, of course, but worth a shot.