I think I should clarify context. I'm not coming from a position of playing the game for a long time, and popping in now-and-then to play with another person. My position is playing through the entirety of the campaigns start to finish with another player, on fresh characters. Out-levelling due to the achievements has DEFINITELY impacted my wife's and my own experience, and made things less enjoyable. It stunted our otherwise flawless pre-achievement experience with an hour or two of wasted time explaining things out of context and figuring out why our stories and unlocks weren't matching up. My wife hated it, and felt like she was doing something wrong because she didn't level as quickly, even though we are playing the exact same content, at the exact same time. How would this be impacting other players in like situations?
We got back on to play, and since I dodge, use mechanic skills and utilities more, and thus finish the achievements more quickly (which reward a LOT of xp). I went from 28-34 from 1 event. I'm still not sure how I got so many of the achievements completed in the single event, maybe it unlocked the backlog as well? It seems strange that someone can out-level the other just because they are dodging more, using more skills, reviving more players, etc.
We've never played another multiplayer/mmo where you could not share the same levelling experience with another person before. While it may not seem like much, the levelling experience is fundamental for retaining new players, and creating re-playability, otherwise no one would have levelling or alts.
Anyway, thanks for the suggestions, so far XP boosting or just putting up with outstripping levels seems the only ways to go without deliberately avoiding content/gameplay.
The adventure guide volume 1 completion avoidance doesn't work. Vol 1 is complete after 14/17, and all of the things except the scouts and emotes are normal gameplay (vistas, levelling, looting, renown hearts, reviving, waypoints, using skills).
Vol 2 has the crafting stuff (+choosing craft discipline, + earn craft levels) you might be able to avoid to stop unlocking vol 3 if you're willing to avoid an entire section of the game (but otherwise will unlock through normal play). Unfortunately it back logs so they're already done for myself. Hopefully it works for someone else though who wants to avoid this problem.