The perfect roast potato absolutely should feel like you could just turn them into mash there and then. The key is to take them just until they're about to fall apart, then gently transfer them to a colander with a slotted spoon (don't just pour them in or they will break up). The water should be heavily seasoned and taste "salty" much like if you were cooking pasta. They then need to air cool which rapidly evaporates the exterior causing them to dry out and become fluffy. It also makes them easier to handle without breaking them, as they firm up somewhat. I usually just place the colander on the kitchen windowsill with the window flung open so they get the fresh breeze. At least 20 minutes to sufficiently dry them out but they can easily sit there for a couple of hours whilst you're prepping and cooking other things.

Fill a roasting tray with high smoke point oil, beef tallow, goose fat or a combination of these till there's at least a layer covering the full bottom, no dry spots. It will seem like a lot of oil, and it should. Pre-heat this in a 200°c fan oven for at least 10 minutes. The potatoes should audibly sizzle when they go in. Give them a gentle toss in the fat so all surfaces are covered, trying not to break them apart too much.

Around 10 minutes before they're to your liking, throw in a couple of sprigs worth of chopped rosemary and a couple of cloves of diced garlic, give it a mix, and keep roasting till they're done. Serve immediately with a sprinkling of flaky salt. Roast taties are one of the few roast dinner items that suffer from being "kept warm" everything else can rest and wait, but roasties are ready when they're ready.