... The AA thing is a non-issue, ignore it.
...
Carrying a USB battery is really no different to carrying AA cells, and using an internal battery frees you from having to juggle AAs.
It's "having to juggle AAs" which is the non-issue. The time it takes to swap the AAs is less than the time it takes for you to take one step forward in the food queue. A USB battery is a lump you must carry all the way round. AAs can be disposed of as and when, or even bought en route as required, reducing weight and bulk better allocated to bonk food.
You're right: used specifically within an Audax, swapping AAs is about as burdensome as plugging into a wall socket at your table, plugging into a powerbank or plugging into an AA-USB charger. The best reason not to buy an Etrex isn't the batteries, it's the crappy small low-contrast screen - unless you're buying one of the Touch series when the best reason not to buy is because they crash all the time. That's the biggest reason why nobody should be recommending them as a new purchase cycling GPS.
Assuming the OP might want to use their GPS outside of Audax though, let's examine a couple of other use cases:
1) I had a long cycle-rail commute on top of a full-time job, I was out the house 12 hours a day. Both for lights and GPS I really didn't want to be getting home in the evenings, disbanding my gadgets to put multiple AAs in the charger, remembering which cells went with which - and then reversing the process before I went to bed because I'd never catch my train if I had to do it in the morning. When light and GPS were both USB powered it was two gadgets to plug into my computer at work - much less stress.
2) A world cycling tour. I ran an Etrex 20 which was fine apart from the crappy screen until it got trashed in Canada, but in many of the countries I've been since then, AAs have been much harder to find than in Britain so I didn't replace it a moment too soon. AAs have been hard to find, but AC power has been available everywhere so my USB devices have remained effortlessly charged. My experience has mostly been in East Asia but I've had the same experience reported from a friend touring another continent.
ETrex 30x permanently backlit and running off the dynamo. Controls as waypoints, map screen set up to show distance & estimated time to next one.
If you're using a dynamo you've solved all the power worries so there are many newer units out there that will be a better fit for you. A 30x is obviously fine if that's what you've already got (heck, I've done about 12,000 km with a plain old 20 within the last year) but don't start recommending obsolete units to new purchasers.