Sorry buddy, but in terms of RAID6 you are out of luck with BTRFS … right now raid5 & raid6 status is “it will eat your data”. I know that there are people that use it and they have no problem for time being … but a lot of them then pop up on btrfs mailing list with “please help me rescue my production raid 5 setup” 
Right now problems that exist with RAID 5 & 6 on btrfs:
- write hole - you loose power mid write (data or metadata) = your data goes bye bye
- scrub issues - yes … it’s supposed to work, a lot of people come back with random errors on that
- parity checksum does not exist - if you data goes bad, you have no idea whenever parity is OK / not OK … so you might be rescuing from garbage parity
- removing a disk with bad sectors can get you stuck … and since there is a write hole + no crc on parity you want gracefully remove disk rather than just unplug it and say “hey btrfs, disk is gone, deal with it”
I know, this sucks and sucks hard … I personally run raid10 for lack of any other option 
Now, this will sound strange - but how much do you care about that data on your video storage server ? Is that a back up (raid10) ? or a hyperfast access where you rotate your data very quickly … because your workstation does not have enough storage locally for storing all data in your projects (raid0 OR raid5/6) ? I know that this is strange but I’ve seen weird solutions for video editing … for even video conversion when you drop video from DVCPRO into SDI stream and then convert it to ASI …
Of course you could go ZFS … but that one requires you to use same size disks … shrinking ZFS is “fun” … your 16GB ram would be cannibalised by zfs driver …