I think that would be a poor direction to take.  The Five is not designed for TV sound and will have an audio lag.

A Port would be a complete waste of money and give you nothing of value for this use case.  If you do go ahead to use the Fives, just use the optical out and a digital analog converter costing a few tens of £s.

You might just as well plug the TV straight into one of the Fives. Note however that this will incur a delay of at least 75ms which, once you notice the bad lip-sync, could infuriate you. Try it and see.

Only the Sonos home theatre products are designed with low delay, so as to work with a TV.

EDIT: John B types faster.

Trey it, as Ratty mentioned,  but the lag may be  more than you are willing to put up with.  It costs you no more than the cable since you already have a pair of Fives.

A couple years ago I tried something similar.  I had a pair of powered speakers and a Zp80 (Connect predecessor.)  I used a cheapo DAC to convert the optical output from the TV  to RCA analog, then connected it to the Zp80’s inputs, the powered speakers connected to the Zp80.  It worked, and I never noticed the lag, BUT, I multitask with my iPad  a lot, listening more than watching, and I am definitely not the best judge on how bad the lag is.  It certainly wasn’t nearly as bad as the lag that Bluetooth sometimes displays - there’s almost a second of lag between video and audio on my car’s Bluetooth for example.

But, it costs you no more than the time it takes to rummage in your cable drawer for the right Y-adaptor from your TV to your Five.

A Sonos Amp and a pair of passive loudspeakers would be the way to go.  The Sonos Amp has a HDMI ARC input so will give you sound the moment the TV is turned on.

Thanks for all your advice. Back to the drawing board.