What helped me to get heel/toe down was practicing on Forza with a Fanatec Wheel and clubsport pedals, that have a load cell for the brakes so it is very realistic. I was trying to learn Sebring, and was also working on my heel/toe, and I must have done dozens (maybe hundreds? but dozens for sure) Heel/toe downshifts and it really helped give me the muscle memory for the right sequence of motions. Prior to this I had the experience that my left would would not release the clutch if my right foot was on the brake still, and so I ended up giving it gas and going into gear as I finished my braking.
Disclosure - I have a pedal extension - I have actually had two - the first one was quite big, and I got to where I didn't need it, so I moved to a smaller one. I have pretty wide feet too.
Both of my pedal extensions were this brand:
SRP Porsche 986 Boxster Aluminum Racing Pedal
The first one I had was the double extension, and then 2nd one was the "regular" extension.