For me there would be 2-2-2 ships with small/medium/large planks:
1.: Raft, "Ramshackle Sloop" and original Sloop: free (the starting ship to travels small distances inside the grid)
2. New: "Ramshackle" Schooner: 1000 gold (less hp, less weight, prefab planks + deck and with maximum 5-6 sailor/animal. No gunports. Its an exlporer ship to the next 1-2 grid, you can use treasure map hunting, or for small scale resource transport)
3. Schooner: 3000 - 4000 gold, the original one, with full customization
4. Brig: 8 000 - 10 000 gold: the original one with full customization
5. NEW: 20 000 - 25 000 gold: as you wrote earlier, something "Frigate like" ship with large planks, bigger than Brig, but smaller than the gally (less gunports, less capacity, less weight and hp, etc.). Typical combat ship.
6. Galleon: 40 000 - 50 000, the original one.
And one more thing: like in Elite Dangerous, probably we could use an "insurance" system. For example:
1. first, you have to build a prefab structure (like the bank, market, etc) for 500-1000 gold.
2. Then you need keep this structure up with gold, maybe with 10 gold/day/ship ammount.
3. Then you need to "connect" or "register" your ship to this structure (dunno, maybe maximum 2-3 ships / structure).
4. Then, if you loose your ship (doesnt matter the cause), you will get a "rebuild token" from the prefab structure where you was connected with. Then you can go to your shipyard and you can (re)build a bare skeleton in the shipyard with the token, and ofcourse without spending gold. (ofc, you need register the ship again in this time)
With this, you can act more freely in the game without loosing everything in a second. Ofcourse, when someone destroys the insurance prefab structure with your ship connection, you need to find a new one, or without "contract" your ship will be gone forever, so you should defend it.
(Sorry for my english, but i hope you can understand)