So... I'm not one for just stomping on an idea and people keep asking for a safe place. So how about one that doesn't break the game mechanic too much.
So, there's a tavern, in the middle of a big island like Plunder Valley, or old Faithful. Boats parked at the island could still be sunk. There's no access to the PL hideout and there are no merchants on the island.
No weapons or guns are allowed inside and are taken from you by mysterious strangers. The route is very long and winding to get to it unless you're A GOOD aim with a cannon. Chests, skulls and crates are also dropped at the front door.
This place is no "safe haven" from being robbed or sunk. This is solely a place for social interaction on the seas.
If you walk out the front door, you could easily get blunderbussed from a camping pirate. This would be a place for like minded crews to meet but not a place where they can run away seeking refuge from being sunk...
FYI I'm not a fan of a "safe place" but perhaps something like this could work?