It's not the Everton deal in isolation that will sink them, it's the wider incompetence it speaks to.
Castore are backed by venture capitalists - which is why they've splashed the cash widely as you say, and they will indeed be 'fine' in the short term - but eventually a return on investment is demanded, and for that you need to be competent.
I've seen nothing from Castore to suggest they are competent. Their supply chain is garbage - as evidenced by these leaks, the dreadful kit quality other teams have suffered - and their inability to get enough shirts produced to release the kit before the counterfeiters.
They don't know what they're doing. Check back in a few years. They'll either have tanked or been bought up by a bigger brand. I suspect the business plan is to survive long enough, with a decent enough portfolio of teams, to make themselves an attractive takeover target.