A couple of critiques of this heater as-built (and I hope this isn't taken too harshly, as the intent isn't to trash the design, but rather to suggest improvements.):
More heat would be collected if you'd left the interior metal shiny, and painted only the PEX. Painting the metal backing and sides raises the temp of the frame, not the water, which only adds to collector losses. Keeping the interior reflective allows the PEX to absorb much of the heat currently being absorbed by the frame.
Without insulation on the wooden sides, the heat that goes into the painted wood is largely wasted since the R value of the wood is only ~R1.5. As built you have about 11-12 square feet of R1.5 (U0.67) sides, and 9 square feet of R6.5 (U0.16) back and ~9 square feet of U0.30 front glazing. The vast majority of the heat loss is out the sides- as it's the most square footage, and the highest U-value. You'd have been better off using only one layer of iso on the back and using the rest on the sides in terms of retaining the solar heat that made it into the box through the glazing, since even with only half-inch iso on the back, the back would still be less lossy than the entire un-insulated side frame. (Using 1" iso for both the back AND sides is still better.)
Double-glass and low-E coatings reduce the amount of solar energy getting into the box, which is why all commercial solar flat panels are single-glazed and not low-E. It would perform better with a cheap single-pane un-coated storm window. Commercial panels use low-iron glazing for a slight performance edge over cheaper single pane tempered soda-lime glass but that's a fine point. The glazing used here cuts performance by at least a third. Going with 9 square feet of ~U1.0 single glazing is lossier sure, but it's the same order of magnitude the loss of the current 12' of U0.67 wood sides. But since double glazed low-E argon filled cuts the into solar gain, the lossier glazing still performs better overall (once you fix the lossy sides, that is.)
By rejecting about a third of the solar energy with the glass, and absorbing half of what got into the collector with the uninsulated sides of the frame (and the metal plate in the back) maybe a third of the available energy gets into the PEX before it heats up and starts losing even more heat, but most of the heat gained is leaking out the sides. I'd be surprised if the net collector efficiency made it to 20%, but it's probably at least 10%. Better home-built & commercial flat panels would hit ~35% collection efficiencies at the outdoor ambient/water temp delta-Ts you were measuring, and much higher at lower collector temps.
But it's the collection of designed in performance reductions that are keeping the PEX below the rated 200F temp with only 6 gallons of water as the buffering thermal mass. With more water-mass/lower-temp operation, more insulation, and painting only the absorber you could probably double the performance of this collector. As-is it won't likely recover the cost of materials in any reasonable time frame.
Without insulating the pipes between the collector and the tank you're losing quite a bit of the solar heat gains as heat dissipated under the house too. (More true of shorter draws than of shower-length draws.)