Seeking upgrade advice: Arc Ultra + Versatile surrounds for outdoor use?

Portable speaker like the Move, Play, or Roam cannot be used as surrounds. 

Removing and re-adding surrounds takes a minute or two.  The radio and network configurations are different for each setup and it takes a while for the radios to be reconfigured and handshakes to occur. Then it takes about the same amount of time to reconfigure as a stereo pair.   Even if a couple minutes each way is tolerable, you will lose any Trueplay settings when you reconfigure, and Trueplay will have to be rerun. 

When streaming a music source (Amazon Music, Pandora etc), a home theatre system will play stereo from the soundbar, and stereo to the surrounds. Keep the Eras configured as surrounds, use them powered via extension power cables, and carry them outside when required. Voila: one “room” playing to two different physical locations, in sync, per your requirement. 

Thanks a lot!. Great answers!!. 
I prefer to keep the Eras configured as surrounds and extend the power cables just for barbecue day.
Now, What Eras do you recommend? I understand the answer should be the Eras 300 as they are expensive, but the price difference worths the improvement over the Eras 100s? 

For indoor use, when suitably positioned and playing Atmos content the Era-300s are the way to go.

I couldn't properly position my 300s and downgraded to the Era-100s and they sound pretty good, not a lot of difference in streaming stereo or 5.1 TV, noticeable but not huge difference for Atmos.

The extension cord idea is not bad but personally I'd look at a Play or a Pair of them, depending on your space. Aside from outdoor use they could serve well in a interior room, bath, kitchen, bed or office. Pick them off the charging base and walk them out to the patio as needed.

This depends on your use case. Atmos music (the Era 300 USP) will probably not work outside (especially when the soundbar stays inside) , so the Era 100’s would be enough. Five’s would also work (but are expensive).

Thanks!!. 

Perhaps I'm trying to do something too complicated. Maybe some surround sound with the ERA 100s and another purely wireless outdoor speaker (what do you recommend) would be simpler and would cost me the same as two ERA 300s. For outdoor music, I don't require Atmos quality, but I do want them to sound as good as possible within a reasonable budget.

I really love my Sonos Play, inside, garage and outside it is a perfect balance of sound and usability.

https://www.sonos.com/en-us/shop/sonos-play

I skipped getting a Roam or Move so I can't compare them but some folks here are discussing that.

On the 100s, as surrounds the SL version will save money at no loss of sound or function.

https://www.sonos.com/en-us/shop/era-100-sl

Remember though: the Play - like all other Sonos portable speakers - cannot be used for surrounds duty, which is one of the requirements. You’ll also need wifi to stereo-pair two Plays. 

You can set the surrounds to play stereo audio, but if the source is atmos music, they will play the atmos audio with the rest of the system.   it should also be noted that you do not have separate volume control for the surrounds and your Arc soundbar.  You won’t be able to mute the soundbar and turn up volume on the surrounds as you likely would want to do for this outdoor patio scenario.  

There is also trueplay tuning to consider.  Your surrounds are part of the tuning, and are tuned to the specific location and direction they are facing.  In other words, don’t be surprised if your surround speakers sound poor when you move them to a different location that we were not tuned for, and don’t be suprised if they don’t sound as good when you again move them to a slightly different location in your living room.

I understand the drive to keep things under a budget, but if OP is considering buying Era 300s instead of Era 100s, a price difference of $500, then the smart option is a pair of era100s for surrounds and a Move 2 or Play portable speaker for another room to be moved outside when needed.

As was said in the first line of the first response.

One would assume the OP has wifi in their home given they are using an Arc Ultra now.