You choose an arrival time at check-in, not boarding time. You can choose an arrival time of 10AM, but that doesn’t mean you will board at 10. FTTF boards after weddings, suites, diamonds, and platinums.
With FTTF, you cannot check-in 16 days out; that is got Platinum and higher guests. You do get priority guest services line, which can be helpful if you don’t like to wait in line.