I ended up contacting Patagonia about my delaminating Torrentshell, and was given a 30% voucher off a new one which is great - once my size is back in stock I'll (probably) get one. I'll make sure to wash it at least annually (with Nikwax Tech Wash).
Patagonia advised that it isn't repairable. And a read across the internet is all of these layered jackets have this issue eventually. There wasn't mention of how to ethically dispose of it, and it's got me thinking about the vast amount of microplastics coming from all these layered jackets :/
To relieve the 'dandruff' effect I washed it, and then used a hard brush on all the areas where it's peeling, and that caused more initially - but got to a point of removing the loose spots and being much less powdery on the clothes for now. Perhaps some sort of adhesive may work to prevent further lifting in the worst areas, but it's going to disintegrate everywhere eventually.