How to level dps alts?

[Urthdigger]

Is doing low level fates via Level Sync efficient?

In general, you want to be doing the highest level content you have available to you. So, after roulettes you move on to your highest level dungeon, and you do FATEs at or close to your level whenever possible while waiting on queues. This does mean you have to actually upgrade your gear as you level, but you can do it.