Sounds like you got some good answers here.
First we do recommend that our athletes do their test outdoors. Typically most will produce more power outdoors than compared to indoors. Can see as much as a 5% difference. However without a powermeter to use outdoors doing indoors is fine. It is all you have. You need the power zones for when you are training indoors.
That is interesting that your heart rate is higher outdoors compared to indoors. Again I usually find athletes heart rate higher indoors than outdoors. Well this is unless it is summer when it is hot and humid. That wreaks havoc on heart rate. Heart rate has many variables so it makes it a big trickier to nail down. Power is instant and absolute.
So to answer your question for when you train outdoors you need to use your outdoor heart rate. Threshold heart rate is what you can hold for an hour. So you can even look back on your training and see what your highest 1 hour heart rate average is and compare it to your test. That should get you close.
Also don’t forget to use perceived exertion. Heart rate will take time reflex the effort. So don’t start too hard on them, unless they are full gas of course. Watch your heart rate as you are riding indoors with power and get an idea how it reacts and what it is in certain power zones.
I hope that clarifies things. M