The US taxes the global income of its citizens, and has punitive taxation for "foreign controlled" investments.
You'd never want to open an investment account in Panama due to these rules (and you'd have a hard time opening an account there in the first place).
You could invest in Panama through US domiciled funds (or a US company), but then of course you're still paying US taxes on everything.
The US is the only country on earth with the power to actually enforce a draconian global tax scheme on its citizens, due to the US dollar being the global currency. If you don't comply, the US treasury shuts off your access to the US dollar.
Eritrea (small African nation) is the only other country with global taxation, but they have absolutely zero power to enforce it.
How much of the 30% US withholding taxes on Dividends from US-equities can be claimed back?
If you can't claim back anything, then it is probably not worth it just because of that.