It's impossible for an American to open a brokerage account in Hong Kong, except perhaps at US brokers like Schwab.

If you've got a HK address and a work visa (and sometimes if you haven't), you'll be able to open a personal bank account, though. Get a credit card, too, maybe. A card with Octopus function that automatically refills will make your life in HK a breeze.

Hong Kong now enforces both CRS and FATCA, so you'll need to provide your US tax number. Finally, note that some banks are notoriously fussy and unwelcoming when it comes to new account holders. Like that big one with branches everywhere.

Of course you can find good professional tax advice in Hong Kong. But as you are about to discover, Hong Kong is a very expensive city. As said, tax advisors in most big accounting firms provide this advice to employees of their corporate clients, with the employer picking up the tab.

Note that if you're working for a big multinational, it may have a tax equalization plan for its employees. Under such a plan, the employer adjusts your salary up or down so that when all is said and done, your net pay overseas is the same as it would have been if you were working in your home country. The employer's accountants take care of computing your tax and making the salary adjustments.