Try using a tray with shallower bowls and you'll get proper bowl-like yorkies. Muffin trays will always make them do this weird thing where they cave in on themselves.
Resist the temptation to open the door when they're cooking, or they collapse.
I've also used a 6" flan tray with great results if you want something you can literally serve the whole meal in!
Not sure what oil you used, but sunflower oil gives them a nice flavour. They need very little oil, common mistake is putting too much in the bottom. I go with no more than 1-2mm depth.
I find letting the batter sit too long has a detrimental effect. I find the sweet spot is 30-60 minutes.
My personal mix is 4 large eggs, 250ml regular cow milk, 200g sifted fine plain flour and a lot of salt. Don't over-mix, it should have good legs on it with < 1 minute of an electric whisk.
I preheat the oil at 220c (420f) until smoking. Then I cook at 160c (320f) for 20 minutes.
Gods I love Yorkshire Puddings. Great work, love to see someone else experimenting with them.


