Eighty five bucks? That is wild. I paid like twelve dollars for a small tub of the fancy stuff at a knife show once and felt dumb about it, so I cannot imagine dropping that kind of money and getting worse results than the three dollar box from the grocery store. Here is the thing that gets me though, you said the first three tries with coil ends just fell apart. That sounds less like a flux problem and more like you were not getting the steel hot enough or you had scale in there messing things up. Borax does not care if it comes in a fancy jar or a yellow box, it is the same sodium borate either way, so the price tag is mostly marketing. It is worth it for damascus where you need the flux to stay fluid longer and not bubble off, but for plain lap welds on mild steel the cheap stuff works fine and you proved it.

Started doing the scrap cut thing after a guy blew out a mitre on some crown last year, saved me way more time than it cost. I also make them show me the first corner before they keep going on the rest. Past that I just let them run and we fix it together if it goes bad, that's how they learn.