Re: difficulty with baking:(
if you know your oven overheats, then you’re doing the right thing by baking at a lower temp and watching it as the bake time approaches. my oven is exactly the same and once you get to know its quirks, it works fine. hang in there and keep practicing! as for pre-heating, i’d pre-heat it at the lower temperature from the get-go. if you set it too high, it takes the oven some time to cool down and whatever you have in there is going to bake at the higher temperature during that while which is likely what is causing your problems.
20 minutes is good!