This may help your decision - the line-in port on a Play:5 is ‘disabled’ when it’s used as a Sonos HT surround.

edit: See this link as confirmation:

Is it viable to move the Play:5 to the Living Room? If not, then I think you may have to decide either to put the TT in the basement, or add the Sonos Port to the Living Room… I assume the Pro-ject Debut has a built-in pre-amp? If not, you will need to add that into the mix… I would certainly test things out using the Play:5 before going to the expense of adding a Sonos Port just for line-in audio purposes.

Thanks for the response. I didn’t realize the line-in feature is disabled when using play:5 as HT rear speakers. Very helpful! 
 

I will test the TT with my play:5 on its own but definitely will be missing the full surround sound

You can either send/direct the Play:5 line-In audio to your Arc HT room via the Sonos App or/and ‘group’ the Play:5 to your Arc HT setup and play the TT audio to either, or both, Sonos rooms anyway.