Side quests are more for supplemental zone lore than for leveling (otherwise they'd be repeatable). The Shadowbringers and Endwalker side quests do scale to level (within the range appropriate for the respective expansion) though it's still not great XP.

If you're looking for an alternate to dungeon spam, FATE XP is sorely underrated by most of the community. I usually get about 1.5 levels per hour if I'm doing them with a small group that's rapidly clearing them. It's not as good as you'd get from a roulette with bonus, but not that far off from what you get from level appropriate dungeons without bonus.

Beast tribe/tribal quests (other than those from ARR) are good XP for the 5-10 minutes needed to complete them if you have them unlocked. They also scale to level within the range appropriate for their respective expansions. Hunt bills (again, other than ARR) also are nice XP for a fairly small time investment if you want a break from the dungeon spam.

Battle leves are not worth it unless you're going for the related achievements. Even the challenge log bonus XP doesn't make up for the small reward from the time investment.