I’d probably chuck in a 4”x4” hardwood post, anchored into some postcrete and tie it to the side of the building using coach bolts at points along the length.

Me, I’d go, get an 8m light carrier pole put in properly.

That would be loads stronger than the poor old ‘D’ that’s holding out for dear life serving mine 🙅‍♀️😂

This reminds me of a similar situation I was in. Fibre needed to run from a post to a single storey building across a garden to replace copper which was pinned to second building between the two. Kelly came twice and said it was impossible even with a hoist. Then a couple of Openreach guys turned up and they had it all finished within half an hour despite having to thread the fibre through a tree and not using the intermediate fixing. Didn't work because BT had sent the wrong router but that is another story!