Jonas, I'm not sure how much you know about shipwrecks and treasure and such. But here is one aspect to your find you may not have considered. These coins were not in some "pirates chest" or even a crew or captain's stash. This is a neatly stacked group of coins. That means they were counted as part of a manifest and loaded into a transport chest, most likely sailing east. If you found this "stack", you can be certain there are others, even if there was a single chest. Combined with the other iron artifacts you have found (I hope you have those in fresh water too) says shipwreck, very close to your spot. May be a little bit off the beach, but very close to your area.
I am absolutely green with envy. I wish I could be there to help you do some off-shore investigation.
Lastly, it looks like the visible date is 1729, which also fits with the partial "Philippus V" visible on another coin. Now, if I remember correctly, you said there are no known wrecks of the period in this area. That could very likely mean that this "unknown" was lost in a storm with all hands and nobody knew where it went down. But you have a date you know it can't be earlier than, and is most likely not more than a few years after. Pretty narrow window to begin searching. Of course, it could be a fluke coin stack from a much later shipwreck, ,but I highly doubt it.