I realized that while I loved Etrian Odyssey IV and Persona Q, they're the only EO games I own/have played. Are any of the other ones on 3DS as good as these? Do they every go on sale?
Persona Q2 is similar to Persona Q but with P5 characters added. If you liked PQ and like P5 and want more, get it.
EO5 is a more traditional EO than EO4 (no dual-classing, no world map), but it has some of the more interesting character classes in the series and the dungeons are well designed.
EO Untold 1 is a remake of the first game and I'd say is the weakest of the 3DS EO games.
EO Untold 2 is really good. Has the story mode option that EOU1 had, but with a more interesting story party (both characters & their classes). The systems are more enjoyable than EOU1.
EO Nexus is like a greatest hits. It uses the dual-class system from EO4 and has a ton of classes (20) and is by far the longest game in the series. However, the vast majority of content is remixed versions of stuff that were in other games. Basically, it's for series superfans.
Personally, I'd say that you can skip EOU1, but every other EO game has something to recommend it. And honestly, even EOU1 is still a good game, it's just that I think the others are all better.
And yes, Atlus usually runs a few EO sales a year.