My suggestion is to combine the washstand with the toilet and shower,
as follows. Make a big hole in the floor to be the toilet and, at the
same time, to be a drain for water. Urinate and defecate into that
hole. Place a shower and a water faucet above that hole. When it is
time to wash your hands, simply use the faucet. For showering, simply
stand near the hole, being careful not to step into it. If you have
extra money, you can place a temporary grate above that hole when
showering, to avoid accidentally slipping into the hole. Keep a bucket
of water handy to wash down excrement and toilet paper after defecation.

i

Truly minimal bathroom however I think the original post only referred to a
minimal shower.