Typically, the Connect IQ data fields would be used on outdoor rides with the Connect IQ Player used for indoor or outdoor workouts if terrain is conducive to performing structured intervals.
You are correct that you cannot use the data fields and player at the same time on the Garmin, but while performing the workout you wouldn’t need the data fields.
The other option outdoors is to free ride to focus type performing unstructured intervals which would use the data fields (no need for the player).
For a simpler approach to getting started with Xert, consider purchasing an NPE CABLE if available in your market area. Compare the cost to upgrading to a BLE/ANT+ HRM.
Then you could run EBC on phone or iPad with all your sensors and create dual BLE connections if needed (ex, Zwift). Or run EBC on phone and watch the remote or session player on your iPad which includes rainbow gauge, data fields, and power chart running in a browser.
The Garmin Connect IQ/Garmin Connect platform add several layers of connection issues that don’t exist with EBC communicating to XO (xertonline). EBC has many more features than the Garmin IQ player is capable of performing, but you should be able to get the job done either way.
Welcome aboard.
Difference between a trial and subscription –
Beginner questions - Support - Xert Community Forum (xertonline.com)