How to prevent overheating?

So far, nothing seems to explain how the water in the tank is overheating. I agree that adding another tank in the summer would improve the situation. The baseline (cold) temperature of the water is about 85F so 24F rise in the course of a day would be 110F. Even a 50% increase in thermal BTUs is 85+36=120F which is nowhere near the 180-190 I get in summer.

Not sure if you are calculating BTUs for SoFla latitude correctly - I'm at 24.5. Just out of curiosity, what do people closer to the equator get for BTU input to solar thermal systems?

Yes, indeed !! And that 120 F number you write of is so far from the 180 - 190 F. you seem to be getting only points out that - as I wrote in a prior post - something in your numbers is simply not adding up.

I didn't calculate the daily tank addition much better than back of envelope stuff.
That was an educated dart throw based on modeled irradiance data for Miami from the Solar Radiation Data Manual put out by NREL for a south facing surface at a 20 degree tilt to the horizontal which shows and August daily average insolation input of 5.6 kWh/m^2/day. which ~ = 1,775 or so BTU/(ft.^2*day) and at an unlikely optimum system efficiency of something like 40% or a bit more. That's what people close to the equator probably can expect for water heating collectors most anywhere. The output is more dependent on solar availability, collector design and collector orientation than latitude.

Maybe the thermostat that regulates the tank's electric heating element is not functioning correctly - or set too high - or both. If the thermostat is heating the water to, say, 160 F and the collector boosts it another, say 25 F, that'll get 160 + 24 = 185 F. water.
Smaller thermal losses in the summer or losses a bit higher in the winter might make that a bit higher or lower.
Can you do a little self help and disconnect the tank's heating element for a couple of days, allow the tank to cool some and see what happens in terms of tank temp. on a sunny day ?

Or, are you sure that the collector area is only 8 ft.^2 ?
That almost seems like a system meant for camping or some portable type operation.
I'm reasonably certain that 8 Ft.^2 of collector surface in So. FL won't get 30 gal. of water to 180 F. unless it starts the day at around 155. Check the tank thermostat.
Any data sheets or mfg. cut sheets you can share ?

While there is a thermostat and electric heating element in the tank, I have the breaker off at all times so no BTUs from electricity. I also monitor my power usage daily and it's all refrig and A/C.
The collector may be larger - what's next one up? 3x5? So 15 sqft vs. 8, or roughly double. No idea who made the collector. The tank is a Lochinvar FTA066KS - 65 gallons (my bad, again, but in the wrong direction). Angle from horizontal is 5 rise and 12 run.

65 gallon now! NO WAY unless you don't use any hot water for 3-4 days.
Check the piping it sounds to me like the the solar heated water is being short circuited within the tank sending hot water back to the collector instead of the cooler water from the bottom of the tank being heated. The stratified water at the top of the tank is being "superheated".
Recommended collector for 65 gallons is 32 square ft.

Interesting you bring that up. I noticed that the solar panel in and out ports on the tank are backwards with respect to the circulator pump. The cold water in and hot out are correct.