Half monty is great for establishing new FTP and MAP values. Sounds like you are heading in the right direction.
Testing takes practice. I’m riding on an indoor trainer for roughly two years and did a few Full Frontals in this time. I am slowly starting to get the hang of it. 
Keep in mind that every test is just a snapshot of a particular day. Sometimes we crush it, sometimes something goes wrong and we are not up to the task.
There is a big thread in this forum about the challenge testing provides ( → right here).
At some point your SUF numbers will reflect and even exceed what Garmin is guessing now. Be patient and enjoy the journey.
I know you will crush it!