In Assassin's Creed Odyssey, the player's level (and gear level) is crucial to defeating enemies, bosses, etc. ever since Assassin's Creed Origins, when the game has gone from a pure ACT to an ACT-RPG.
However, the main questline alone yield little XP to make progress in the game. Thus, it seems like it is nearly necessary for the player to participate in side quests, conquest battles, etc.
The above wasn't an issue back then since AC was just purely ACT. You may consider me idiosyncratic - I like AC's main storyline, but am not as interested in the quest storylines. Some quests do correlate with the main storyline (e.g. Cult of Kosmos) but most of them have standalone plots which are not intriguing to me.
Here is the question - are there any pragmatic ways to level up without playing side quests and such? Otherwise, which kind of quests (or anything) give you an ample amount of XP?