Have you considered doing a heart rate drift/AnT-test by yourself? I’ve done that a couple of times over the last year and I really enjoy possibility to retest for checking progress without spending the extra money of lab visits. Regardless of the method, those results should guide your is-this-base-big-enough question. Sounds to me, though, that you have enough endurance to be able to execute Zone3 and 4 sessions without problem, given your race/long uphill repeats this fall. That said, if you do haven’t planned any race in the nearest future and your goals are towards the longer side of the spectrum, I guess the benefit of ME would still be great. Maybe you can fit in some weeks of ME while waiting for a spot at a lab?