Thanks Chris, made sense to me. I even sub out the triads triad. Makes things even more funner. I break rules all the time, but there is a logic to the breaking.

An inverted Gm7 is a i ii iii or vi. A Bb6 is a I or IV.
Bbmaj7 to Bb6 is very common for 4 bars of Bb. Many more subs are often used to relieve even that boredom.

A C7 plus (13, #b,9, 5, etc) are dominant or leading tones type chords. (as we've discovered, it's still dominant to me) Pure and simple.

As for IV chords. I prefer to look at them as sub-tonics, rather than subdominant. It's just a preference. Once you add or imply a tritone, it's dominant.