Too high an FG is not good. I see you have looked at a couple things as possible suspects but you are looking at the wrong items. You do have an appropriate amount of grain for that Original Gravity so that is not a problem. A fermentation temperature is 75° is too high, but would tend to lower the FG. Too high a fermentation temperature can cause formation of fusel alcohols which will affect flavor, though I don't think this will be a problem at only 75°.
A mash that is too thin will cause conversion problems, but this will lead to a low original gravity and unconverted starches in the wort, not a high FG. Likewise, too much water in the sparge will not affect the FG.
The main thing left is the amount of unfermentable sugars left in the wort. In all grain batches this will come from an excessive amount of crystal malts in the recipe or from too high mash temperature. If you can post your grain bill and let us know your mash temperature, we can probably nail down the source of the problem.
One other thing that sometimes leads to a high FG is racking the beer off the yeast in primary too early. Some older references emphasize getting the beer off the yeast as soon as possible to avoid the burnt rubber taste that develops when the beer sits on dead yeast too long. While this can happen, this process takes 6-8 weeks to happen, while some folks insist on racking as soon as active signs of fermentation begins to subside, often as early as 4-7 days. This leads to a number of problems including high FG and unwanted esters. I seldom do a "secondary" anymore, but almost never do it sooner than two weeks after pitching yeast to the primary. I normally don't rack or keg until after 3 weeks in primary.