I know that
@jpflavin1 walked in April and encountered high snow drifts. I have heard him describe the day crossing from Poladura to Pajares and there is no doubt it was very challenging.
I searched to find some of his accounts, and came across other threads describing early Salvadors. This is
one. I would search the forum using the terms Salvador and April and see what you get.
I have asked Cuñarro/Ender about this many times (he is the one who did all the original marking and publicizing of the Salvador, almost singlehandedly). His advice has always been that it is not common for there to be too many snow storms in April, with the now de rigeur qualification that with climate change all bets are off. But more typically, the issue may be accummulation of snow in the mountain stretches. And there are places where the snow drifts could be dangerous. The approximately 16 km between Buiza and Arbás del Puerto can only be described as remote. No car access, no pueblos.
I have walked this route in June, September, and October, and hands down June was the winner. The vast expanses of yellow with the mountains as backdrop make it one of the most beautiful stretches on any camino I have walked. But it is always gorgeous.