Standard hole saws will not work for drilling rock so you need to rethink that plan.
There is also a danger when using rocks with holes that are too small as adult fish can become trapped in small holes.
I would just wait until you can find a place to buy cheap rocks rather than continue to spend money on the aquarium types you've been buying. I don't know where you are exactly but there has to be a commercial place that sells rocks, large stones and small boulders that would much cheaper and more natural looking plus give you options to change up the placement of any rocks.
yup,
did you see the pic up there?
got a large makita and various concrete bits, have 4”currently, used to tie into storm water...
Obviously would not use a wood bit on concrete....
this action below...

