False Bottom in Conc Pool: Feasible?

My pool is 40 x 20 x 12' IG concrete or gunnite, some kind of troweled masonry surface around 40 years old. It is pumped down pending repairs and repainting.

Wow, now that's a hole!!!

My question is, how feasible would it be to fill in the lowest part to around 10' depth, or even 9'? I can see some advantages: less volume of water, better circulation, a positive effect on pump and filter sizing (these are both being replaced anyway)... bringing the main drain up to the new elevation won't be a problem.

One friend who knows about pools said I could just dump sand in to backfill the void, which makes OK sense to me, but what should I do for the new finished surface, and how would I tie the new masonry bottom into the existing side walls and be assured of a leakproof joint?

The thoughts and comments of this enlightened community are always most welcome. thx

Put in a vinyl liner??