Bit late here but I think you just need to go hotter. And you don’t actually need as much oil as you think, just a decent layer so everything thing can get coated, but they don’t need to be swimming like some have said. I set my oven to 220-230C 428-446F the oil doesn’t realistically get that hot once the potatoes are in there/ opening the door to move them about, but you need that temp set to keep ahead and get thing frying, and not be waiting 2 hours. I find if I bring the potatoes up from cold to a good boil and cook for 7-10mins if works out well, drain shake and let dry off, I often do them a few hours before I plan to cook so they dry right off but it doesn’t matter too much, a few tablespoons of polenta or semolina sprinkled on them for the shake is good too. The pan should be hot before they go in and don’t turn them until the bottom is crispy, probably the 25-30 min mark, then you’re on the home stretch. I find they take about 45-60 mins, it means you can take roast meat out and wrap in foil and towels then crank the oven up for potatoes and make gravy and whatever while the potatoes are cooking and the meat will still be hot for serving. This methods works for any potatoes from my experience.