Best way to acheive backups

Hi Guys

Hope i have this in the right section but here goes..

I am trying to work out the best way to store and protect my Buisness data backups. I currently have a Readynas 422 mapped to a local drive and my main business data files and complete PC backups are stored on the 422. It's configured as a Raid 1 redundant array with 2 Tb of storage (2x2Tb drives).  However because of the mapped drive it does leave the NAS open to ransomware attacks should anything get through my PC antivirus. I understand the risk is small, but i've recently seen it happen to another client of mine and so i'm keen to prevent it. They completely lost all of their data and backups etc (Yikes!).. 

Ideally the solution is an offline backup of my Nas system.

To that end i had thought of buying a simple USB portable drive and doing a manual monthly backup of the nas drive. 

However i do have a slightly older readynas DUO thats not being used and i was hoping i could put that into service as my secondary backup. The DUO runing raid 1 would give me double redundancy and i'm hoping to completely automate the process so i'm not having to remember to run the backup once a month.

The DUO does have some limitations though, mainly data rate..  It seems that the backup jobs are run at about 1/3 the normal data transfer rate (presumably to allow overhead for regular user access while the backup is in progress)..  It currently runs at about 6 megabytes/s. To back up 1.8Tb  of data will take something in the order of 83 Hours!  

So is there any way to speed up the backup job on the DUO?

Secondly once a full backup is done, does the next backup (assuming i schedual it for once a week/month) just do an incremental backup? Or is another full backup done?  Seems crazy to do a complte backup every time when only a small portion of the data will have changed..

I plan to set the power timer to power the unit up, run the backup job,  and then power down again..  This would keep it switched off and dissconected from the network most of the time and therefor help keep it isolated from any ransomware attack. Can someone tell me,  if the backup runs longer than the set power up time, will the unit  wait to complete the backup job before it powers down?

The thought occured to me to have the 422 "Push" the backup onto the DUO (rather than have the DUO "pull" it from the 422) as the 422's data rate overhead is much larger, but i then have to time both units together and also allow enough time for the backup to complete meaning the DUO is exposed on the network for longer than necessary. 

Any thoughts on the above?   Is there a better way?

Many thanks

Lee