Archival storage for RAW video I am filming with my BMPCC4k and figured out to be my workflow as follows:

Import Clips to harddrive.
Create Proxy's using Adobe Media Encoder.
Edit in Premiere Pro.
Then Color Grade in Davinci.
Then replace Proxys with Color Graded video.
Export.

I am now thinking about how to archive those videos. The budget of my projects don't allow storage space of multiple TB each.
So I was thinking about storing rendered videos, so that we can at least make slight changes afterwards in editing or export diffrent formats.

What would be the best codec to use to export the video? With a good compromise of quality vs space. I would say no more than 50mb/s.

I am using Windows.
If your system has external SAS, you can either buy an older LTO Tape Drive System, such as an LTO-5, or rent one. Or go for an LTO-6 or 7 drive. 

Linear tape really is the best choice for a number of production environments which is why so many post houses have adopted it for footage back up. 

The tapes have large capacity, are enterprise secure, have built in fault tolerance, are fairly easy to use, and it's the CHEAPEST way to archive. 

An LTO-6 Tape will hold 2.5TB uncompressed content (raw video), will write at speeds of roughly 140mb/s and the tape will cost you about $20. 

A 2TB HDD Bare Spinning Drive is far most costly than that and will not run/be locked/greased up if you try to use it 50 years from now. 

They are archival for 30-50 years. 

It's what the National Archives uses.