When leveling my main job I purposefully skip all sidequests that don't unlock something and focus exclusively on the MSQ. All those skipped side quests I do on alt jobs for shots of xp here and there.
The primary sources of XP for my alt jobs though are:
Squadron Dungeons, Challenge Log, and daily Leveling Roulette (healers only) 1-60
Daily Main Scenario Roulette, Challenge Log, Beast Tribe Dailies and daily Leveling Roulette (healers only) 60-71
Trust Dungeons, Challenge Log, Beast Tribe Dailies and daily Leveling Roulette (healers only) 71-80
I don't like tanking group content so I stuck to mostly Beast Tribe Dailies and Challenge Log stuff for those jobs.
I am pretty terrible at most of the DPS jobs, am not a fan of melee in general, and don't want to wait for long queues, so Squadrons and Trusts are amazing for leveling the DPS jobs. I do hit the Main Scenario Roulette on DPS jobs for 60-71 though since that's a dead zone for Squadrons and Trusts.
I love to heal (I main WHM), and am more than happy to run leveling roulettes on any healing job.
I hit fates from time to time when convenient but I don't care for fate farming. I still don't have the ShB shared fate stuff completely unlocked. 60 fates per zone is just too much to ask of me. I think I've done maybe 10-15 fates in each zone so far.
Deep Dungeons are fun in small doses but I can't chain run them too much cause it gets too repetitive and monotonous for me.