I just spent 4h+ collecting an ashen key and an ancient vault key (purple triangle thing) as a solo sloop. Then I got completely demolished and humiliated by a larger crew on a Brig before I could reach the vault.

Yup. You could have reached the vault if you went the long way instead a bee line toward it. Maybe even scout around for any “Large ships!” On the horizon…they don’t just sneak up.
Even if so. Rowboats are your friend as a solo

All are equal on the sea. You can take a brig and galleon if they also aren’t paying attention.

You think someone of “equal ship size” won’t do the same thing? It the same affect being sunk and stolen from

Also for giggles. Those commendations aren’t going anywhere. You have the rest of your gaming time to do them, maybe post a help me ad? (Looking for group)