i commend you on striving to keep the water quality acceptable, but looking at this logically; if you have to do 30% water changes a day, just to stay at 10-20 nitrates; this suggests to me the bioload is far, far too heavy.
I can't suggest an estimate or idea of what the ideal parameters are in terms of bioload, but to give an example on my side, bioload in planted tanks etc is such that I could neglect to do a water change for 6 months and be a-ok, and while the argument may be that this is due to smaller fish in smaller set-ups etc, if anything this would strengthen the argument on having a balanced ecosystem that can mostly balance itself out with appropriate sized tanks. The set-up in question would destabilise, in what; 4-5 days? Very, very risky and not advisable imo.
As others stated, id go down the route of a breeding pair; if you go do two 100 gals with 3 Oscars in each, I'd strongly expect a pairing then subsequent bullying, killing of the third weakest.