Can vegan diet and HIIT workouts move the needle?

Hello! I am so pleased that I found this group. What an incredible wealth of information it contains. Here's my story and I would appreciate any feedback on whether my plan of action sounds like a good one. My name is Steve, i'm 69 years old, and weigh 180 pounds and an 6'. In February I had a yearly check-up and my doc saw that my PSA was 3.9. He drew some blood and did a K4test, They came back with a PSA of 4.8 and a 8.3 which put me at 9.6%chance of aggessive prostate cancer. He suggested that I see a urologist and see what he thought. The urologist said to come back in September and get another PSA test. I asked if we could get an MRI and he thought that would be unnecessary. After the appointment, my doctor asked me how it went and he said that if the urologist won't order the MRI, than he would. The MRI came back with a gland size of 5.5 x 5 x 4.5 A PSA density of .136
There is a far left apex lesion measuring 0.8 x 0.8 DWI score 4 CCE negative T2 score 3 PI-RADS 4
impression:
The pI-RADS 4 sub centimeter lesion is in the far left apex peripheral zone. No visualized seminal involvement or definite extra-capsular disease.
I went back to the urologist and he thought it probably wasn't cancer but he would do a biopsy. Here is the important part of the results.
Part 6- Prostate left medial apex, core needle biopsy: Prostatic adenocarcinoma, Gleason 3+4=7 (grade group2) involving 40% 0f 1 of 3 cores. All other cores were ok.
We then did a Decipher test and it came back .46 which is .01 away from the high point of low risk. (.45)
I meet the urologist again and he told me just go home and live your life. You can do Active Surveillance and in September we will recheck PSA and in March he would do a follow up biopsy and another MRI. I asked about diet and exercise which I had been reading could help the situation but he did not think I could do anything that would change the outcome. I spent hours and hours reading everything I could and decided that i would up my exercise to HIIT workouts as well as really step up an already very active life and I would also transition to a vegan diet since everything I found showed that plant based diets could have an impact on PC. I began the diet on May 18th and i'm doing great on it, obviously missing meat, cheese and dairy but if it changes the picture I will be pleased. I lost 15 pounds and leveled out at 180.
I have 2 questions for this group - First, do you agree that active surveillance is the way to go with this small lesion and second do you think the diet and exercise program can have an effect on lowering PSA and possibly shrinking the tumor? I sure appreciate your time and insight!