Your's is a most interesting project as normally the foundation crazy quilts are not really quilted but tied.
But after looking at the picture and reading your description, on first thought I might suggest the following.
Do the motif quilting in the sashing. The vines would be good and easy. Even tho the sashing is different width, I would keep the same motif for all. This would connect the blocks together.
SID around each block and then SID within each block but selectively. The blocks need interior SID quilting.
Then select sections of each block for the motif quilting.
For example the separated heart in the center could have a feather motif. What I'm saying is that the old crazy patch quilts had embroideried motifs scattered around them. Do the same with your quilting. Make these as detailed as the space will allow. Scatter the leaf shape from the sashing throoughout the blocks.
And definitely start in the middle. You could do all the SID around blocks or sashing first. This would baste the quilt and would make the other quilting easier.
Just some thoughts.