Timber Fascia board exterior

Planed is just rough timber put through a planer machine to take the surface down a few mils and give it a nicer finish. What you need to do is treat it with a good quality preservative, ideally let it soak in the solution for a day (then dry out) and then try replace the whole roof timber in one length (how's your relationship with your neighbour?) to eliminate that cut as it's where the timber rotted from exposure.

Then for added points install a plastic strip over the upper face of the timber and silicone it so that the water runs over the timber, not behind it; then paint.

Just go with PVC fascia the whole way then? You can trim the lip off sections of fascia to give you the angled sections. I redid mine in PVC and it's less work as there isn't any painting.

It's just cut-down fascia board. Can't seem to find sheets or lengths of it otherwise.

Brooks or Chadwicks are my local suppliers here, but any of the providers.