Creative, nicely done 
I think the only thing I would do differently would be to use JavaScript to calculate the streak, rather than calling an external script. I’d also pass the dates as integers - just makes them easier to work with as you can do direct numerical comparisons. Another option would be to transpose the list of dates into a Helper Table, and then figure it out from there. But that’s probably a bit of overkill, and you’d need enough rows in the table to cover the maximum streak length. So not really a good option.
Side note: I suspect the reason why you couldn’t use greater than/less than with the streak count is probably because your script returns the number as a string. If you run that through a Math column (multiply by 1) that should coerce it into a number.