Perhaps the LG G5 isn't what you're looking for in a phone. No big, LG also has the V10. It's bigger, has a secondary display, and now it has Marshmallow on AT&T. The UI won't change to match the G5, but you'll get all the usual Marshmallow goodies.
The OTA clocks in at around 1GB in size, so you'll want to find some WiFi. This is Marshmallow, which includes Doze Mode, granular permissions, Google Now on Tap, and more. This OTA also has bug fixes and security patches up through March. This is 6.0, not 6.0.1, so you won't get all the new emoji.
You might not get the update notification or be able to manually pull it right away. Give it a couple days. AT&T only lets you check for updates once every 24 hours, so you can either start messing with your clock or just wait patiently. I know, lame.
AT&T has updated its V10 status page with the official OTA info. The changelog is below.
- Upgrade to Android 6.0
- Video Calling (IR94)
- Advanced Messaging (IPME)
- Zach Barrett