Always on , data sync in parallel or serial

Hi , We have 2016 SP2 CU3  enterprise version awlays on configured. DB  contained some varbinary, image  type data tables  which DB expecting to grow annually 2TB. probably will store pdf bills with the DB.  expecting to remain 2 year data with archive plan.  so questions 

Q1.can we manage  the DB with such a big size with always on sync. 

Q2. Is that alwayson sync is parallel or serial . 

Q3. We plan to have backup on primary as expecting secondary may have delay  with data. in this case  if not setup backup from secondary , then L drive get fill up  often. What is the solution ? setup log backup and full backup on secondary  site as well?   

any one can help we great. 

regards