So any other player type attacking non-hostile ships?
I suppose any such action should technically turn you pirate.
That's exactly what it would do. If you're not a commissioned officer attacking the enemies of your nation or a privateer attacking the enemies of whoever sold you a letter of marque, and you're not defending yourself against someone who attacked you first, then you
are a pirate. (The distinction between privateer and pirate is a little vague, the nation you're attacking may regard you as a pirate, while the less scrupulous privateer will just regard a letter of marque as a way to commit piracy while keeping at least one nation friendly.)
There should perhaps also be the possibility of not following any play style at all.
For example if you just follow a storyline or make your money through treasure quests or something.
We can consider that some sort of "freebooter adventurer" type thing.
That's what I've always liked about PoTC (and a few other games, starting with the old space trading/combat game "Elite"). You aren't constrained, you can do what you like but there may be consequences to face.
So there are two things to consider here:
1. When should the pirates accept you as one of them?
and
2. What advantages should there be to playing as a proper enemy to all nations? And how would this remain interesting?
In fact, how would you even survive at all without being able to make use of the resources that large towns provide?
1: Any time. If you have any letters of marque, you lose them. If you subsequently crave forgiveness from a governor and then buy a letter of marque, you lose your standing among pirates. (Possibly log that fact, then the pirates will never accept you again because you've betrayed them once already. Or you have to pay lots of money to crave forgiveness from them, re-join and lose your letter(s) of marque again.) Fame should not be a condition, you're joining in the hope of becoming famous!
2: Target rich environment.
Anything you meet on the high seas that isn't wearing a black flag is a target. Beyond that, it's an alternative way of playing instead of serving your choice of nation(s) as a privateer, smuggling or honest trading.
It may be possible to crave forgiveness from a governor and
not buy a letter of marque, so you retain your standing with pirates but refrain from attacking ships belonging to that nation, then at least there are some ports where you can trade. Alternatively, you can sail into port under a false flag and, provided you're not recognised, trade with the store and shipyard. This will become harder as you become more famous, of course...
"Brave Black Flag" starts you off as a pirate but not actively hostile to anyone except Britain. Perhaps that's the way to go - when you first join the pirates, you're asked who is after you, then that nation alone becomes hostile. Either you're seeking refuge from that nation or you're seeking revenge against that nation - as it's free play, you can make up your own background. Provided you don't then attack everything in sight, you can remain neutral to one or more nations so you can still use their facilities. (When I played "Brave Black Flag", I victimised Britain, Portugal and Holland, but left France and Spain alone. So provided I wasn't stupid enough to enter port under the pirate flag, I could still use French and Spanish stores and shipyards.) Otherwise the facilities of Nevis pirate settlement, Turks Island and possibly Tortuga are at your disposal.
In the final analysis, the problem of how to survive when all nations are against you isn't for the programmers to solve. It's for the player to solve. That's the challenge of being a pirate.