Due to variations in your local terrain, distance and elevation gain will never go according to any plan. The plan is intended as a rough guide to see what a possible macrocycle would look like. You’ll need to adjust it (as you’ve done) for your particular circumstance.
If you want to retest (AeT or AnT) then the end of a recovery week is a good time to do so. However, know that similar to training sessions, frequent tests may or may not show continuous improvement. Usually, improvement fluctuates with a general upward trend over the long-term.
If a base session feels like something you could do day after day, then it’s a good sign that it is the right intensity and duration.
You don’t have to avoid gain entirely during a recovery week. Go by how you feel. If your legs need a break, then minimize the gain. If they’re fresh, then add some.
I can’t remember if the free TP account calculates drift, and I’m not familiar with the Garmin app, but neither is necessary.
During a drift test on a FLAT course, warm up gradually for at least 15 minutes until your HR stabilizes at what you think is AeT HR. Then hold that pace for 60′. Press the lap button halfway through. So when you’re done, you’ll have one 15′ lap for the warm-up and two 30′ laps for the test.
If the average HR in the 30′ laps is within five(ish) beats, then you can compare the average pace. Divide the average pace of the second lap by the average pace of the first. Likely the second half will be slower. If the pace drops off by less than 5%, then the pace and HR should be near AeT.
Unlike an AnT test, an AeT drift test will feel much easier for most people. So feel free to test it more frequently.
Aerobic Self-Assessment for Mountain Athletes