I’m sure that the Twinzer will fit the bill as far being fast and loose is concerned.
I had a 5’8" Larmo Twinzer a while back which was definitely a fast and loose board in small mushy stuff and still went ok in steeper waves but had to be ‘nursed’ a little in bottom turns when compared to a keel fin fish of the same length.
However, it was pretty foiled out in comparison to a keel fish of similar length that I ride and had modern shortboard rails, meaning that it struggled to float me properly, so that may have affected how it surfed for me. The placement of the fins is also farther up the board than other twinzers that I have seen/ridden, such as a couple of Minamis and Hynson’s twinzer. With hindsight, I think that Larmo’s Twinzer could be ridden a couple of inches longer than what you would normally order for a keel fin.
I’m 6’1" and 180lbs, but I guess I ride boards that are considerably smaller than you would ride by the sound of it. I would have loved to try a 5’11".
I think you should give the 6’6" a go, particularly if you are looking for a board that will be fun in stuff around the head high range and under.
With a 6’6", you may find that it will go better in bigger stuff because of that extra rail line, but for me, the board I had really went better in smaller waves.
Maybe drop Larry an email and ask him what he thinks?