Upping mini dachshund’s prozac dosage

Hi guys! I have a 12-13 pound dachshund named Toffee that has been 10 mg prozac (generic) for about two months now for fear based aggression towards people and other dogs. While it did improve his behavior outside and allows him to focus more during training, he is still very much over threshold when we are in a new-ish environment. For example, we went to an isolated trail during the non busy times we used to go to before his aggression started last week and he would not listen to me when I tried to get his attention. While he didn’t bark at everyone who passed by like before (still at some), he was pulling and very overstimulated. In our neighborhood (and weirdly the airport), he listens to commands and loose-leash walking but that all goes out the window the moment we go outside of the neighborhood or even when someone walks on the same sidewalk as us. I have had professional training with a trainer that specializes in fear based aggression and Toffee did super well in the neighborhood but once again, very much over threshold anywhere outside of that. We have tried just sitting at different areas while he takes it all in but he just ends up cowering and can’t focus. Because of all this, I contacted his vet with concerns and they recommended to up his dose to 16 mg of Reconcile (brand name for prozac). The vet even suggested 20 mg but I want the ability to downsize if the higher dose is too much and I think a 10 mg jump at his weight is way too high.

For doggo parents who use 16 mg reconcile, how has your experience been? Has anyone have any tips on additional training I can do with the new jump in dosage? ( BTW I am currently working on Proximity based training right now with the trainer)