If I were in your position I'd do a 50% water change, cleaning the substrate and filter as well as I could, without adding the Thrive and wait 24 hours before testing again. There should still be some NO3 in the water, but cleaning out as much organic gunk as possible will give you a good baseline to then modify your dosing, if needed. Do a normal dose, wait 30 minutes, and test again. That'll tell you about how much NO3 your normal dose adds so you then have an idea of how much to add to reach your desired target. Since things are looking pretty good in there now, I'd aim for 20-30ppm and give it a couple weeks to see how things respond.
If you prefer lists.
1. Clean the tank and filter well and wait 24 hours to get an approximate baseline of NO3. Make sure to use old tank water when cleaning the filter, we need the to be "normal" and not sterilized.
2. Test nitrate after 24 hours, do your usual dosing, wait half an hour, and test again to get an approximate idea of how much you're adding so you can figure out what to change if needed.