Pretty sure garmin do take other factors into account for their race predictions as they’ll update when your lactate threshold does, even if your vo2max remains the same.
I think you are right regarding lactate threshold but im not sure how they factor in running economy as it's difficult to calculate. You need an accurate VO2 measurement. I guess they use HR as a proxy but in practise I find HR isn't such a good measure of metabolic output for various reasons; hills, unrelated stressors, heat, aerobic decoupling, etc.
Well.. a crude estimate of relative running economy is pace/heart rate and tracking it over time. Seeing the number go upwards means your economy is increasing and vice versa.
A fair amount of people track this because it's a feature of pretty much every training/coaching app.
