Is it ABSOLUTELY definitive that NO Samsung Smart TVs passthrough 5.1 via the optical port?

My 65" JU6700 (2015 model year) can definitely pass through 5.1 from HDMI sources and out the TV's optical port.
Not sure about your model though. My suggestion is if you can purchase the playbar locally from somewhere that has a return policy to try it out.

High Drive,

...Or you could output the Dolby Digital 5.1 from the TV via the 'usual' HDMI port and cable plugged into a 5.1 DD audio extractor device, which will then output the audio in DD 5.1 format, via a toslink optical connection, to the Playbar.

Use a device similar to this one ... https://www.reichelt.com/?LANGUAGE=EN&CTYPE=0&MWSTFREE=0&CCOUNTRY=447&ARTICLE=164186&PROVID=2788&wt_guka=22596754217_79177333817&PROVID=2788&gclid=CMysvvXjtMwCFe4y0wod1S4DXQ

I'm sure there are plenty of other HDMI audio extractors mentioned here on the forum and elsewhere online. I'm not sure which ones work best, or are good value for money etc; so it may require a bit of research, but at least you can then get a TV to suit your own personal requirements, with all the other features you require.

Thanks but my first/best hope is of course to hook the Sonos system up to my existing TV without any additional devices or expense. Trying to figure out if that's possible before considering alternative options.

That's okay HighDrive, I just thought that if you were not able discover if the Samsung TV would pass the DD 5.1 (from HDMI to toslink) to the Playbar and you had to purchase a Playbar to try it out .. at least the HDMI DD 5.1 audio extractor, might solve any problems encountered and at least that would mean you could carry on using the Playbar regardless, rather than returning it.

I would like to see a new Sonos Playbar myself with an HDMI input port, in the not too distant future, (wishful thinking), but at least there are plenty of options around now to overcome any limitations on the connections between the 'ever-changing-hardware' that we each encounter from the different manufacturers.

The HDMI DD5.1 audio extractor, I thought, might also help to expand your options when choosing which new smart TV to buy.

At least for a long time, Samsung TVs did not send 5.1 audio from HDMI inputs to the optical connection. I see that Samsung is now making soundbar models that support surround sound, so it's possible that at least some newer Samsung models might be sending 5.1 through the optical connection now.

I find it is better to get the audio onto Toslink upstream from the TV on the HDMI. I usually get less sound latency as TVs usually have a delay on the audio out or the Sonos has a small delay in digital to analog conversion.

On one TV, I use a Monoprice 13345, but there are many cheap devices that will do this. On my other TVs, I come right out of the Tivo, but lack a remote control audio control.

I have a samsung Series 7 smart tv 2018. It happily sends DD 5.1 through optical to my Playbar. Both from HDMI connected ATV 4K and smart tv apps.

My new Samsung QN75Q6FNAFXZA passes DD 5.1 over optical without issue. My older Samsung TV UN60H7100 did not and I had to use an EDID manager to force it to.

I have a 2018 Series 6 Samsung and can get DD 5.1 via the optical connection.