To follow up on this, I've been using a Taotronics TT-BA07 bluetooth transmitter and some bluetooth speakers.
For the boat I use a pair of Amazon Tap speakers. These are versatile enough otherwise (for all their Amazon Echo features) and they're also decent Bluetooth receiver speakers. The BA07 pairs with both of them simultaneously.
For the backyard I use a second one of those adapters and some ION portable units. One is a Job Rocker Plus, the other a Job Rocker Max. I use a second Bluetooth adapter to avoid having to re-pair the devices.
This takes audio from the 3.5mm headphone jack on the Dashbon. I don't detect any noticeable latency. I'm sure there is 'some' but it's not enough to be detectable when watching movies.
Bluetooth adapters: (the TT-BA07 is the one to get)
The bluetooth adapter has it's own battery inside. It could be powered on it's own, but I don't. I plug it into the USB port on the Dashbon and power it from there.
For video I use an Amazon Fire TV stick streaming through a WiFi hotspot on my phone. The Fire stick I power using a USB battery pack, as it wants to sometimes pull just a bit more juice than the projector wants to provide. I've use a splitter to power them both and sometimes it's worked. But for the sake of not making the movie watching a chore I find it best to use a battery pack.