The application fee is a one time 10,000 baht fee (it actually varies depending on which embassy you apply through). You can enter Thailand as many times as you want and stay for up to 180 days each time without any additional fees. I have already entered twice but stayed less than the 180 days.
If you wish to stay in the country and not leave within the 180 days then you can apply for an extension of an additional 180 days. No one has done this yet because 180 days hasn't passed since the inception of this visa. There is some debate on what this extension will cost due to some wording in the English and Thai language descriptions of the visa from the MFA. Many assume it will be the same 1900 baht fee while others suggest that it might be 10,000 baht. I personally believe it will be 1900 baht but no one is 100% sure at this time. The only reason the extension fee will matter is if you don't plan to leave the country within 180 days.
For me, the extensions are a bit irrelevant because I will travel out of the country occasionally and reset the 180 day count for free each time.
It isn't 180 days per year... days in country for the calendar year is irrelevant for the visa (but not for other things such as taxes)
You get 180 days each time you enter Thailand. You can enter as frequently as you like.
So in theory, you could stay up to 180 days, leave, come back and stay again up to 180 days and continue repeating this for the full 5 years. Stay 1 day, 20 days, 74 or 180 doesn't matter.
Likely possible if you have a business license, federal tax EIN, and write a letter from your company that states your persoanl responsibility, your monthly income, and that you are allowed to work remotely to complete your role. Sign it.