Really enjoying watching the build progress, making me want to do something similiar.
One thought that did occur to me having seen your propose build, would be to make one corner board (lower left of you general plan) a y-piece with a point, presuming you could maintain the smooth curve you have on those pieces.
The intention being to allow you to place the buffer module off the point when set-up in full 'loop' mode, as it seems a shame to not use the module if you have built it, and it would make a nice waiting/reversing position in operation.
The other benefit of a y-piece is it could be used to make a return loop when combined with the 3 other corners, so you could set up the layout as a long out and back; Fiddle yard - station - reteurnloop. Could be useful setup option at shows etc.