Interesting… I should have read this topic before. My Boltardy seeds (King) were rubbish as well. I sowed a second time as I hoped I did something wrong in my first sowing, but again, as good as nothing. I sowed Chioggia beetroot at the same time, with exemplary germination rate (sourced from well-known e-bay vendor).
I sowed a few onion varieties, Santero being among them and also being the only one that failed miserably (Suttons). I sowed a second time, just to make sure, but it looks as if the experience is going to repeat itself…
Best germination + vigour for onion seeds are for seeds I purchased from Bingenheimer Saatgut (Stuttgarter Riesen, https://www.bingenheimersaatgut.de/) and Semailles (Rouge de Huy, a local variety, https://www.semaille.com/en/).