There's this new shiny hunting method in USUM where there's a 7 percent chance to find shiny in the Ultra wormhole, I know this is true as I found one on my third try, so, if I save before encountering it then soft reset, if it's not a shiny would that work?
No, once you enter an ultra space wild it is predetermined if the Pokemon in said wild is shiny or not. This means that if the Pokemon you encounter is not shiny then it won't matter if you soft reset, but if the Pokemon is shiny then you can soft reset all you want and it will still be shiny but its nature and IV's will be different each soft reset. Hopes this helps
If you want quick shinies go for ultra worm holes if you want to hunt more Pokemon go for sos and if you want an ultra beast or legendaries go for the classic soft reset it varies on what Pokemon you want to hunt.