Congratulation on making such great progress. Consistency is the key. I’m glad you are reaping the rewards.
AeT HR will definitely plateau at some point although elites can have an AeT at 85-90% of max HR and 95% of AnT.
But even as your AeT HR plateaus your pace at AeT can continue to improve for years. The longest span I’ve heard of is 10 years of steadily improving AeT pace. You’ve just scratched the surface my friend!
As an ultimate limit look no further than elite marathoners. The marathon is an event that is competed as ones AeT or just slightly above that metabolic point (usually around 2.5mMol/L of blood lactate). Top marathon men are running well under 5min/mile for 26 miles and the world record marathon pace is just over 4:30/mile. I think you’d agree that this is a heck of an AeT pace!
The sky (your genetics actually) is the limit.