- Bisher war alles in Ordnung (example from Duden)
- Unsere bisherigen Bemühungen waren vergeblich = "Our previous endeavors were in vain"
- Bisher war das immer in Ordnung = Until now, it was always ok.
- So weit, so gut (colloquial, common saying) = "so far, so good"
- So weit kommt es noch (colloquial, common saying) = (sarcastic) "this will never happen"
- So weit das Auge reicht (colloquial, common saying) = "as far as the eye can see"
- So weit ich weiß ist es schwer, deutsch zu lernen = as far as I know, it is difficult to learn German
- Bisher dachte ich immer, es ist schwer deutsch zu lernen. Aber jetzt habe ich ja einen Duden.
Note that in combination with bisher, past tense is often used as it means something similar to "until now". (Duden: "von einem unbestimmten Zeitpunkt an bis zum heutigen Tag, bis jetzt")
So weit can mean a distance (see example 4), or in example 4 it also means "so far" but does not necessarily refer to a distance, (similar to "so far").
So in both, you have a comparison as something which happened previously (until now or until here) and often inferring that this is now changing or may no longer be the case.
Bisher, I would use in the sense of "in the past and until now, previously".
In the example 7, you could not replace "so weit" with "bisher" - it means "as far as I know" - meaning I am letting you know my knowledge is somewhat limited. But not that my knowledge lies in the past.
I hope this is helpful. I get that it is not so easy to decide what to use.
- Refer to "Duden" to get the exact definition, like in a dictionary
- Read, listen to people etc. to get the "living" examples