I have found it easier to do all of the final sanding before assembly.
There are always hard to get to places that are not hard to get to
before assembly. If you pad your work bench, and all clamps, the only
sanding you will have to do will be touch up work. I used to do a lot
of oil finishes, and found that it was also easier to finish before
assembly. I would always find one place where I didn't get all of the
excess oil off, and it always seemed to be in a corner. I would cover
all glueable surfaces with the blue tape, and press it down very well.
You could also save cutting mortices and tenons until after finishing,
but I found that the machining would leave too many marks that needed
to come out, so I do that first. With spraying, I don't know. I am not
an expert at that. If you have any floating panels, you may want to
spray them first so that all of the wood is covered, and later
shrinking won't expose bare wood. This doesn't sound like it will be a
problem with what you are making.