would be a good wattage of bulb to shoot for and where could I find one?
Look for a 1500 lumen bulb. Ignore the wattage. Watts is just a measure of electrical consumption. It doesn't tell you how bright the bulb is. Lumen is a measure of how bright the light is.
The other issue you might run into is will the brighter bulb fit in your aquarium hood? You might have to do a lot of looking before you find what you are looking for. Based on your picture that is your biggest problem to solve. You might have to scrap the hood and purchase a seperature light.
Nutrient issue a deficiency or too much flourish?
Difficult to tell if it is a deficiency. Flourish comprehensive is one of the better fertilizers out there. Most fertilizers are diffident in at least 3 or 4 nutrients. The flourish recipe is week on the macro nutrients N/P/K/Ca/Mg/S but covers most of the trace nutrients other fertilizers miss.
Check your water harness GH. That is combined measurement of Ca and mg (calcium and magnesium. for N your typical aquarium nitrate test kit will tell you what that is. Ideally you want that between 5 and 15ppm. Plants will handle high nitrogen levels well but fish are more sensitive to it. As for the rest the are some P (phosphorous/phosphate) test kits available but some are pricy. Hanna instruments sells a good one HI713 for $50. K test kits for fresh water are hard to find. S (sulfer/ sulfate) test kits are extremely hard to find. The ones I have found are very expensive.
If your GH is low (120ppm or less) A GH booster. Sachem equilibrium might be a good idea it would add Ca/ Mg/ S and some K without doing any harm as long as you keep your GH level very close to your current GH (That means you will probably be dosing very small amounts 1/64 teaspoon or less). My phosphate generally is low so i check that with my Hanna meter and add some to keep it between 0.3 and 1ppm. The phosphate fertilizer also adds some K.
Since you have been overdosing for some time I would not worry about trace levels right now. In any case you would need to spend $1000 dollars to get test kits for most of the trace nutrients. It is just not practical to test for them. Just reduce your dose to a little below recommendation to allow water changes and plant growth to consume the excess. And than after a month or so bring the dose to the recommended level.
