I levelled my now lvl 50s simultaneously. This is my formula
I crafted them lvl 5, lvl 10 and level 15 and level 20 gear sets (green) so as they level fast at first I could change their gear and keep levelling. Training gear gives xp. Once they out grow gear sets put into bank for other alt to use. After level 20 i crafted blue gear.
I focused on doing harborage quests
I joined mages guild, fighters guild, undaunted, thieves guild
I picked up wayshrines and skyshards and books aswell as mats while doing my psijiic questline
I did delves for skyshards too, they not hard to solo.
I also did dungeons as soon as i hit lvl 10
Did pvp training quest to get my rapid manouvres skill that gives me 30% speed boost which helps when farming
I also levelled my provisioning and all crafts on all my alts just by cooking food, doing writs and deconing gear i found in chests or from dungeon drops
I now have most of all my 12 characters levelled to 50 and with most skill lines nearly maxed out. And nearly all riding lessons maxed out. It is fun if you focus , relax , enjoy.
Good luck and enjoy the journey. That's the fun part 😊