I sympathise with your question. In short, STICK WITH IT.
We’re of a similar age and looking at your treadmill results we have a similar HR profile so my experience may apply to you, particularly if like me your prior training was mainly in the “harder is better” camp.
I’m approaching the end of year 2 using UA protocols and it did take some time to see improvements. My first AeT (UA’s treadmill test) gave me an AeT of 110 and boy, was it hard to go slow enough to stay under 110, so I quickly gave up biking and used the gym treadmill as my main training vehicle for most of the first 6 months. During that time I did move my AeT up to around 120 but like you was getting frustrated. Further AeT treadmill tests were giving inconsistent results and I was trying to force-fit AeT improvements into the results. Scott Johnston (UA founder) quite firmly pulled me back – at my age (now 65) an AeT in the 120’s may well be as good as it gets, but I should see speed gains with that. Roll on to now ….. my AeT is still in the 120’s, but I am fitter, faster and stronger than I have ever been in my life, both on the bike and in the mountains …. full stop.
I looked back at my records and averaged around 9hrs training a week for the first 30 weeks with >90% of that time at or under AeT, and most of the time above would only have been by a few beats – I did zero high intensity sessions during that time, and to this day low intensity sessions are still the mainstay of my training. Net, stick with it and try to get weekly volume in the 8-12hr range – it will work, but the adaptions take longer at our age, and everyone is different.
I could go into a lot more detail, particularly tweaks I have made in year 2 indirectly due to COVID restrictions, but the principle of having low intensity as the mainstay still stands. The exact definition of low intensity is where protocols differ, particularly if you have a large gap between AeT an AnT that due to age cannot be reduced to the desirable 10bpm. Discussion for another day.
Hope this helps.