About the optional nature of „da“ in the original example:
There are often ways to omit conjunctions.
What may be relevant to consider:
Alles was Weed tun müsste, wäre am Leben zu bleiben, da sonst die Quest fehlschlagen würde.
Alles was Weed tun müsste, wäre am Leben zu bleiben. Sonst würde die Quest fehlschlagen.
That is two main clauses (verb second).
As an example of the pathway leading to complex constructions, I often see two main clauses connected with comma + „deshalb“ ‚therefore‘. That is (still) wrong in Standard Written German but apparently many people (including myself) consider this a viable complex construction.