So it's obvious that portals are challenging.
Having an open world so large requires fast travel.
However, you want people to explore and find the rare randomly seeded stuff.
Right now the system is odd… Metals can't go through a portal… At first that seems okay but turn your metal into metal tools, weapons, nails, etc., and no prob. So that's weird.
Also, once you've completely dominated a region and are just using it for it's resources, it takes forever to sail back and forth clearing it out. The system unnecessarily punishes people who have earned the right to get their materials more easily.
So here's an idea.
Instead of basing the use of portals on the presence / absence of metals, what if portals were powered by runes?
Ores and metals and everything else could be taken through a portal, but the portal is activated by a rune in your inventory that gets destroyed when used.
The first runes could be dropped in boss fights, then found randomly in crypts and as you get more powerful they become less rare and once you are at a certain level, you can craft them yourself.
That would make them very precious in the early game because you would always want to hold onto one in case of emergency. Then in mid game you would have a way to earn them here and there and you would save them up to use in transporting large quantities of metals at once and finally in the late game you would be creating them and wouldn't be limited anymore in teleporting yourself or materials wherever you want.