Unfortunately washed up..
After a short inspection, i confirmed it hadn't ingested plastic or anything man-made, it had no oil on its feathers and no visible fishing lines tangled anywhere on it, therefore it most likely died of natural causes. I only noticed some outer primaries that were broken at the middle (pics 8/9) and (on the same wing) there were a couple missing secondaries and secondary coverts. May be signs of scavenging if not related to the cause of death. Nothing too concerning at least. Must have been dead for a week or so when i found it because it was infested with large maggots and carrion beetle larvae (along with adult ones).
Another thing to note is that the secondary and primary covert undersides were full of mite eggs (5-30 per feather - example on pic 7) and some tail and wing feathers had holes caused by adult feather mites.