Let's cover it for you! August is a good month for your plans, and at each of the destinations you will most likely be either resort or boat based.
So expensive! Most of the areas are based on Reef breaks! Mentawais some resorts have softer and even some sand breaks, Maldives are not in general as powerful as the Mentawais. Fiji some beach breaks but most on offshore reefs.
Because of the time of the season you need to book well ahead.
A Mentawai recommendation Kandui Resort, the access at least 25 breaks with a number of speed boats that take you to waves of your level, pros and frothers get to go to other breaks. Expensive but worth it.
Maldives mainly Resorts, two spring to mind Hudanafushi ( Louis) and Cinnamon Dhonivelli ( Pasta Point)
None of these areas have just rock up,and hang out at the beach , it is only resort or boat!
Then think of travel, wherever you choose is entirely dependent on the whims of the ocean, it can exclude you by playing the three bears , too big, too small,and just right. Hopefully the latter.
You need to consider, flights to each and board transport, adds to the cost, Garuda at least includes boards.
An absolute must, specific travel Insurance, that covers full medical evacuation, reputable resorts or boats will not accept you without it!
I've surfed all these areas, Kandui Resort tops my list ,guides, safety, personal photography, and waves available for your ability, with room to upgrade your Ability.
Expensive , yes about $3000 US per person for 10 days and an two flights and luxury speed boat trip to cross the ocean to the resort!
There are other resorts in the same area to compare!
Hope this helps, can advise you more if needs be!

I've taken up troll hunting just for fun, instead of a rifle I'll just use a pun! 冲浪爷爷