What $20K application fee? I think that all the government fees combined for the Panama friendly nations visa tally between $1,000 and $1,500.
Panama does not allow dual citizenship, but currently does not enforce that law. As everything else in life, that could change.
Panamanian citizenship will never happen unless you live in Panama for a number of years and establish basic ties to the country. That important point is often glossed over by many OSPs.︀
Legally avoiding taxes is certainly possible, but not guaranteed and is based upon your fact-specific︁ circumstances. A legal residency is different than a legal tax residency, which does not automatically︂ follow from legal residency. You must separately establish your Panamanian tax residency.