For the first two or three weeks that SoT was out, I was strictly solo slooping (because my friends who planned to play didn't have it yet, though even now I still solo sloop frequently when my friends can't play) and sometimes I was sunk by fully crewed Galleons. However, there were also times where they sank me, took my loot aboard and unknowingly picked up a hitchhiker along the way (I have an hour long video of me hiding in a Galleon's bottom deck until they all left the ship to do a voyage ashore. While they were gone I stole their ship, rammed it into another island nearby, hid all the loot and then sent the galleon off on it's own to sink. After that I mermaided to my ship and returned to the island to pick up all the loot).
Just yesterday I was playing with a mostly full galleon crew (3/4, our fourth friend decided not to play after we'd already gotten the galleon stocked and started). A solo sloop went by Thieve's Haven where we were doing the Athena Run and wound up parking behind some nearby rocks (though we knew he was there). I went to board and deal with him while my crew were seeking the treasure only to find his ship unmanned. As I started stealing supplies and torching it, he arrived at our ship and caused a little trouble before he was killed. When he respawned on his ship I killed him before it sank and mermaided back to my ship thinking that was the end of it.
I was wrong.
He managed to make it to Thieve's Haven with no supplies, sneak by my searching crew and any skeles and got up to the ship's ladder. I heard him coming and tried to shoot him on the ladder but he slid down to dodge and got aboard before I could switch to my sword. He managed to kill me and then the rest of my crew when they returned. When we respawned we killed him and aside from 4 pineapples and some wood he didn't manage to get anything from us.
What's my point? Even though we ultimately killed him (and we've been playing since the first month of the game so none of us were new or inexperienced), one player managed to take a 3-man crew even when we knew he was there. We got overconfident and he knocked us down a peg before we took him out.
Every situation is different but many people will tell you (as I've just illustrated with my own stories) that a full crew does not automatically mean you'll lose as a solo slooper. You will have to think ahead of your foes and know when to run instead of fight but anyone can manage to take out anyone in this game.
Sometimes you'll just have to hide below deck for a while to wait for your shot and sometimes you just come on like a madman and slaughter everyone you can.
I truly hope that you can become a better solo slooper or find yourself a good crew to play with because the PvE server that many request is just not going to happen (and I don't want it to). It can be discouraging to lose a lot of loot sometimes but you can beat damn near anything the game throws at you (except for geysers when you're trying an X-Map in the Roar, those will most often beat you because they just don't want to leave the dig spots).