I like keeping things simple, especially communication between athlete and coach.
Feel is an excellent tool that doesn’t require any device or download. If it feels hard, back off. If it feels easy, go a little longer at that effort or a bit harder for the planned duration.
To see one’s maxHR and estimated LTHR is not difficult, as this software shows one’s history, as shown by @david here. Testing is also a simple process, but there seems to be a fear of testing (or seeing a result that is below the expectation), or a shortcut option to get a result expected from a longer effort.