Which speakers do you want to play, the 100’s or conventional speakers wired to the Pro-ject amplifier?

hi - I'm using a pair of Era 100s for all output

I think I might need the Pro-Jject MaiA S3

https://www.project-audio.com/en/product/maia-s3/

I knoew nothing about the pro-ject amp, does the owner's manuals discuss connecting it to  a Sonos system?

You realize the Eras are not able to be connect to an amplifier? They only connect over wifi or using the Sonos input adaptor to a 3.5 mm line level signal.

Any of your source components will connect to the Eras, one at a time, or you can add an audio switch to allow you to select from any of them.

I'm connecting the Eras via the Sonos line-in adapter (RCA > 3.5mm jack), just trying to source a phono switch (integrated amp) that fits with my system. I don’t want a clunky 3rd party switch, but that’s really all I need I guess.

If you are not wiring conventional speakers to the Pro-ject amplifier, I suggest retiring the amplifier and using a selector switch for the SONOS Line-In.

Note that Line-In is simply a network resource that will not automatically play through the physical speaker where it connects. Line-In can be selected to play through any combination of players.