a) only benefit of filing I-765 is its free and if approved it works as combo card for both EAD and AP
b) I could be wrong but I think l-864 is required for all AOS application except Employment-based

But for I-765 it says filing fee is $485 it’s not free.

You don’t have to pay fee for i-765 and i-131 if you file under c(9) category which is adjustment of status.

Source:
https://www.uscis.gov/sites/default/files/files/form/i-765instr.pdf
Page 23



Exceptions
Initial EAD. If this is your initial application and you are applying under one of the following categories, a filing fee is
not required for:
“12. (c)(9) or (c)(16) Any current Adjustment of Status or Registry applicant who filed Form I-485 on or after July 30, 2007, and paid the appropriate Form I-485 filing fee. If you file Form I-765 separately from your Form I-485, you must also submit a copy of your Form I-797C Notice for Form I-485, as evidence of filing Form I-485 on or after July 30, 2007, and payment of the appropriate filing fee. If you did not pay the appropriate Form I-485 filing fee because your filing fee was waived or you are exempt from paying it, you must pay the Form I-765 filing fee or request that the filing fee be waived; and”