Speakers for the deck- okay to discuss here?

A couple JBL Charge speakers would do the trick. They put out tons of good sound, are rechargeable, waterproof, and you can easily link multiples together with a push of a button on the speaker. They have long battery life and you can connect to them via Bluetooth or a cable. I carry a single, smaller JBL Flip on my company truck as it fits in a cup holder and it does a good job on a jobsite. 

I have a pair of JBL Flips and my dad has a pair of the JBL Charges. The flip is more compact and the pair does nice for background music on our smaller patio. The Charge is slightly larger and works better for my Dads larger outdoor area. The sound from the charge is more full with better bass response for sure. I don’t miss anything necessarily when I use my Flips but the charges are definitely a worthwhile upgrade. Both models sound shockingly good for the size and the cost in my opinion.

I would personally rather have two of the  Charges than a single Extreme for the same cost. And while I am a fan of external DACs on my main rig, with these speakers I wanted simple and portable. If you add a DAC to a single speaker you won't get its benefit on any other speakers you pair up wirelessly. That alone makes it a non starter for me. 

I pair a couple speakers together and place them on opposite sides of my patio (off the ground is helpful) and Bluetooth in with Tidal or Qobuz from my smartphone and it works great for barbecues and get together. If we want to listen critically we hit the main system inside. 

Also, YouTube has a ton of comparison videos on the Flip vs Charge vs Extreme if you need a better idea on size of these units and other features. 

I bet you will like the extreme. If you do just buy a second one and pair them together. Nothing wrong with that!