Can i use my laptop as an external recorder
Not really. Not unless your laptop happens to contain a 4K capture card.  Most laptops don’t have video signal inputs.  You can do HD with a Blackmagic Ultrastudio Mini Recorder for a little over a hundred dollars, as long as you can find capture+encode software, but putting the encoding onus on your laptop isn’t very reliable.
If recording space is your major concern, many cameras are now capable of writing directly to externally attached SSDs, presuming the data rate generated by the codec you’re recording is lower than the write speed of the SSD.  I’m not sure about the R6, but if it doesn’t, you might consider trading it for one that does, depending on your priorities.