Then you are facing a losing battle. Maybe 14 years ago here in S.E. Michigan we lost all our Ash trees to the borer. In the city of Troy where I worked, the city had planted them exclusively throughout the city, especially the highway medians. On a daily commute I never really noticed them but then one day I did as they were all almost dead.
The residents in the city who had fully grown adult trees that were now dead or dying had to pay hundreds of dollars to have them cut down.........
Regarding the chemical treatment you mentioned, my neighbors had a pretty little ash tree in their back yard that they were constantly treating in the way you mentioned but it eventually died too.
Since many people here in the southern half of Michigan have vacation properties in northern Michigan, the state was warning everyone to not transport cut trees used for firewood to their properties up north......
The ash borer larva eat their way through the inner bark making many intersecting tunnels cutting off water to the leaves. The chemical treatment makes one wonders if a process similar to the production of bt corn for the ash tree is practical? https://entomology.ca.uky.edu/ef118