Recommendations for multi-room setup?

The Node 2i is indeed what you need to connect to your existing amp and speakers. I would definitely use powerline adapters though as the wireless performance is problematic for some users (including me). I am using the cheap TP link powerline adapters with my Bluesound to resolve this.

You will certainly be able to stream spotify via Airplay 2 (iphone, ipad and mac) to the bluesound and even the sonos (synced multiroom). However, Roon cannot multiroom on both sonos and bluesound. You can use Roon to stream to multiple sonos (even multiroom) OR stream to multiple Bluesound (multiroom) but not on both (in sync).

In summary, for multiroom you can work with the following combinations:
Roon multiroom: Bluesound + RPi OR Sonos
Airplay 2: Bluesound + Sonos

If instead you opt to adopt Sonos all around then that will also work well (Roon / Spotify via Airplay 2) and it might be simpler (certainly cheaper). You will not be able to employ Roon DSP but the Sonos Trueplay will work. The new Sonos Port will serve well as long as you keep the latency low (i.e. avoid connecting it to an AVR to ensure that when you engage multiroom the music remains in sync).

Do you want to be able to play different sources/music in different rooms/zones at the same or the same music in all the rooms/zones at the same time?

That’s definitely some food for thought. Our amp/speaker combinations are significantly better than our Sonos speaker pair, so rather than going further with Sonos I think we would go in the other direction – maybe replace the Sonos with something that would be simpler to interface with the other gear.

Clearly this whole realm is more complex than I was initially hoping. I think one of the most central points is what works with Airplay 2, since we’d be using Apple devices for control. So, as previously mentioned, it sounds like I need to do more reading to understand that better.

Pretty much the latter. The rooms are close enough to each other that trying to play different music on speakers wouldn’t work too well.

Or just get a Deon Heos multiroom streaming amp.
wire all your casual and outdoor listening speakers to this. Keep your hardcore 2 channel main setup standalone