With the small amount of time I get to play the game, as well as the fact that I never touched Arena, there's exactly 0% that I will ever get 100% in the game. Even if I for some reason did have the Arena achievements all done, or didn't care to include them in my total, I personally don't find grinding Achievements for Achievements'-sake to be something enjoyable. No shame to those who do, though! It's a great goal for some people to have, and they get a lot of enjoyment seeing those things get checked off their list, and more power to them. For me, though, since I only get a few hours per week to game anyway, if I WAS going to 100% a game I wouldn't be playing Sea of Thieves.
Mostly I see what activities I want to do and see if there are any commendations/achievements based around them, or go for things with cosmetics that I like, like Umbra's Chronical Tattoo Set or the Curse of Sunken Sorrow. I'll never get the Gold Curse because I don't really care for it all that much and it's too much work for me to do solo (I might go for Curse of the Order, though. That fits my character's aesthetic pretty well).