Yeah the side quests are definitely used as padding to artificially prolong gameplay and add grind/gaps between the story missions. Even though players have the "choice" to leave side quests for later, the alternatives to leveling without side quests in between main quests and rraise your level are very much grinding. I never noticed it.
Investin skills that fit your playstyle and make the game fun. Side quests can be tedious but most take 5 min to complete. There is around 130 quests including main. If your going for 100% completion save all trinkets until 100 before selling Chain assassination has saved me many times in the arena.
Belowis all the information you'll need to beat Family Reunion, one of the Siwa Side Quests found in Assassin's Creed: Origins. It's rated for a level 3 character. This mission is acquired via
MBWUK.