Gotta be a better way to get a person back aboard than rigging a block
and tackle.

Why not attach a swivelling block to the stern rail so that a line can
be quickly put into it with a large carabiner on the end for hooking
to the MOB. The line runs through the swivelling block (which is
permanently mounted) and then to one of your winches (cabin top jib
winch would be easiest for me, jib sheet winches for others), then you
just put your winch handle in and crank away.

Use the same idea but attach the swivel block permanently to the end
of the boom. Then when it is needed, put the line into it (this has
to be an opening style block so the line can be easily put in it) and
run the line to a winch. With this method, you have a better angle of
pull.

Same idea but mount the swivel block high on your backstay, another
turning block on deck to give a fair lead to the winch... Mount the
backstay block and turning block permanently. This enables the MOB to
be hoisted high out of the water.

Using the backstay enables the system to be used when you are "Hove
To" so you dont have to run the engine to stay in place thus
endangering the MOB.

Perhaps simply use the poly line permanently attached to your
LifeSling and place it in the opening blocks after the MOB is in the
LifeSling.

Using a winch is second nature to most of us sailors whereas a block
and tackle, hmmm.