I've pretty much given up on the idea of hiring senior devs, who cost a fortune and still (in my experience) take ton of training before they're useful.

I'd rather have junior engineers who I can train hard over 3 years. They're productive within a few months if you're doing it right (and have chosen people with the right attitude). Treat them well, train them well, give them pay rises parity with their increased skill and they'll stay with you.

It's much more valuable; you end up with a loyal and experienced team. Incredibly rewarding to see people go from completely raw to world class talent, too.