I think you are over thinking this. 
All you need is a ballpark signature to start with and move forward from there.
HRDM is for when you have sufficient power, cadence, and HR data on file to estimate XSS and Focus for rides that lack power but include HR and cadence.
If you lack the required volume of historical data (months worth) you should disable HRDM.
Power is king and that is what you should be recording for the majority of your activities indoors and outdoors along with cadence and HR if you want HRDM to work well if needed in the future.
You will know if your signature is “off” if recommended workouts seem too easy or too hard based on their difficulty level and intensity. When you have sufficient data on file you can also request a signature review with support@xertonline.com if you still doubt the numbers.
Any of the BT workouts will suffice to test your signature periodically. You will be warned if no BT events in more than 3 weeks. Try them all over time and see which are your favorites.
Otherwise just ride and accumulate more data while XATA learns about you and you learn about Xert.
I imagine you have already reviewed this thread but in case you haven’t –
Beginner questions - Support - Xert Community Forum (xertonline.com)
Related: Huge difference between SYSTM/Garmin & Xert - #20 by ridgerider2