Since you said that your Jackdaw isn't that strong yet, the best way for you is probably taking out schooners and low level brigs. As you have already mentioned, it can be difficult finding ships with metal, but they do exist, promise!
I found that the hunters which come after you when raising your wanted level are a good source of metal.
In your circumstances, I would do the following:
- Raise your wanted level to level 1, this will cause a level 17 brig to spawn and hunt you.
- Incapacitate the ship and board it to receive the full quota of loot. Repair the Jackdaw if it gets damaged.
- Fast travel to another location. This will cause another hunter to spawn.
- Repeat steps 2. and 3. until your wanted level rises to level 2.
- A level 20 brig and a level 8 schooner will spawn and hunt you.
- Incapacitate both ships and board.
- Fast travel to another location.
- Repeat steps 5. to 7. until your wanted level is level 3 or close to it.
- This is probably where you want to decrease your wanted level. Best way is to board a ship and lower it by one. But if wanted level 3 ships spawn and you don't think you can handle them, you can always bribe the local officer to reset it completely.
I find forts are the best places to fast travel to. They have a harbor master (in case you need to repair, sell loot and so you can upgrade whilst farming) and a local officer (for bribing) right next to the dock. The disadvantage of a fort is that you will have to fight away from it. If the fort sinks the ship, you won't be able to board it, nor will any loot spawn. However, an advantage of being so close is you can run to the fort when things go bad.
Try to always board over sinking it outright. It may feel like a chore, but schooners and brigs can mostly be cleared just with the swivel cannon. You also get the full loot instead of half, and a chance to repair the Jackdaw or lower wanted level (or keep the ships if you need them).
Make sure you sell the rum and wool (and any extra wood or cloth you aren't using, as you don't want your stores to fill up and waste loot).
Hopefully that helps you with your lack of metal. When you upgrade sufficiently, you can farm higher wanted levels and therefore receive more loot!