I am not a Bach scholar, so take this with caution.

My understanding is that we don't have much to know about his private life. There aren't many personal documents of his... For other composers we have many letters, and even fragments of conversations. We have many accounts of them because they were celebrities in the big fashionable cities, in a time in which artists were deemed important, but this was not the case at all for Bach.

What you describe ("fairly religious and perfectionist," "kind of secretly wild and temperamental," "a fatherly if dull figure") are indeed not very descriptive, rather stereotypical, ways to describe the life of a person. As you say, there is a very romanticized view of him, created in the time of Great Man theory.

As others have said here, we know almost nothing of his personal life---other than his birth, death, who he was married to, who his family was, and who his children were. I personally believe that the inner man emerged through his music; as Gardiner put it in his book Music in the castle of Heaven: "Perhaps the man could only express himself through music, as any other means of social interactions may have been a cause of stress for him.*

Not verbatim, just as I remember reading it and grasping its gist.

Reports say he was welcoming to guests and loved being with his family. As a teacher, there are reports of him looking down on anyone who did not take music seriously. At one scuffle in a church where he was auditioning to be an organist, a fellow contestant played so poorly on the organ Bach flew off his wig and launched it at the performer, claiming he would be better off as a cobbler.

I don't think Bach was a vain fellow, as he is reported to have said, "All one must do is hit the notes at the right time, and the instrument plays itself." We know he regarded his music as being very difficult to perform----he expected the voices and flutes to do, difficultly, what his hands could do quite easily.

I can only think that his personality and personal self are woven into his notes. People say the Baroque era was not one where composers inflicted their personal feelings onto the keyboard, however I disagree with that statement; there is no way we hear the second to last chorus of the St. John Passion and think that Bach was simply sitting around, enjoying a cup of wine, and not weeping, thinking about a child he had buried that day perhaps. Death was all around him, and perhaps because it was, he had a finer appreciation of life. He could, at any moment, die from a myriad of reasons in those days.

Disclaimer: I'm no expert, just a lover of his music.

Bach hated authority, and these roots go all the way back to his youth. He would skip school, his grades were awful. Then his parents died and he was basically on his own----sure his older brother may have tutored him in music but was he as loving as old Ambrosius? Was he as comforting as Elizabeth? I doubt it.

Bach didn't even think his music would last beyond his lifetime. His cantatas were perhaps expendable, much like how we view modern pop music; nothing survives a year, or a few months.

Bach is said to have been buds with Silbermann, a fantastic organ-builder in Thuringia. He is known to have played a few jokes on the man, one time playing a chorale tune in the key of Ab minor and claiming the key sounded horrible and his work was awful! Bach is known to have made organ builders shudder at the thought of him inspecting it; he would pull out all the stops and play massive chords to test the organ's durability and lungs. Often he would write a prelude/toccata/fugue for it (this was perhaps the motivation behind the toccata/fugue d minor, bwv 565).

All I can close with is that his music is both challenging from a performer's point of view but it pierces my heart to no end.

I...don't know what else to say, but I hoped I helped.