Hello and welcome!
Key questions you’re asking and I can shed light on some but not all…
Personally, I found the indoor treadmill test to be easier to do. More ability to control variables.
Treadmill test = 10% grade
And in case you haven’t already seen the below articles:
Indoor Treadmill AeT test – https://uphillathlete.com/8-diy-steps-to-figure-out-your-aerobic-threshold-indoor/