If you have a source that can provide Dolby Atmos, then I would also upgrade your surrounds to Era 300’s.  I have this setup and the audio is fantastic.

The Arc is a good place to start, it will work with your existing Sonos. The 300s would be next up for me

Looking at your profile I noticed you have a Play 5 Gen 1 listed as one of your speakers. I don't know if you use the S1 app for your system right now, but when you upgrade to the Arc, you will have to use the S2 app to control your system. The Play 5 cannot be added to the S2 app. 

I just wanted to mention that before I make my suggestio.

I upgraded from the Playbar to the Arc last Christmas, and while the Arc is a great upgrade in terms of filling the room, it doesn't sound as good as the Playbar. The Playbar does sound “warmer” compared to the Arc. Even after i did Trueplay, i decreased the treble on the Arc to -2 to reduce what I can only call harsh highs. The Arc does outshine the Playbar by filling the room with sound and giving new life to my old Sub and Play 1s.

With that said, I would upgrade the soundbar before the surrounds, especially if you have a TV that has an eARC HDMI port. Dolby Atmos audio was the deciding factor for me. I love the Playbar, but the Arc has given me more satisfaction.