I started playing this game in January 2019 on the recommendation of a friend, who played with me the first month or so then discovered fleet servers, and I was left solo slooping. I am by nature mostly antisocial (fleet server? NO) so have now spent over 90% of my time, 75 DAYS playing, solo slooping. My buddy played (and plays) with me occasionally and helped me grind the last few levels to PL a couple years ago.
This game can be tough on a solo sloop, but is eminently doable. I voyage and emissary for all factions (75 OoS, GH, MA; 24 PL), except Reapers (although I am currently 55 in Reapers) knock over regular skelly forts, sea forts, and skelly fleets. Eyes on the map for those pesky organic and portal jumping Reapers, eyes on the horizon and head on a swivel, what's that other boat out there up to and is he minding his own business, etc. If I really want to be alone I head to the Roar. It's all part of the fun.
Last Friday I finally finished Legend of the Sea of Thieves 98% solo. At first skelly boats meant a trip to the bottom (skelleons mostly in 2019, even in a sloop); after a while I learned how not to get sunk. I never looked for skelly boats, but if I got jumped I could sink them most of the time (except skelleons). Soon I started hunting skloops, but not skelleons (they are still mostly an automatic trip to the bottom even for an experienced solo). I avoided the skelly fleet battle and skelleons until about two months ago, when I realized I wanted LotSoT. I started looking for fleet clouds; skelleons still are a still a pain, but put a few rounds into them and they tend to veer off. Skloops take a few and veer too; they take 4-7 and will eventually sink. Skelleons will sink slower, but eventually they founder with a few more shots. I LOVE boarding them with firebombs and a blunderbuss when they ram or come too close as they sink me. I plan to get sunk multiple times, so don't stock a whole lot of stuff, knowing my boat will respawn nearby the cloud and will have forty cannonballs plus what I have on me and what is on the island. Unless I get interfered with by a Player, and it happens every now and then, I can knock off a fleet in less than an hour and collect TONS of loot to sell at Daggertooth or Reapers (it's usually around 50-70K). The 500th was a skelleon and I used a just-found megakeg after boarding him as he was sinking me; it was EPIC.
Solo slooping? It CAN be done!