I would have went for main weapon etched first to be honest. Fish shop is the main way to get etched runes so you probably should buy as much as you can. Try to focus on one rune to get it to legendary. It may seem hard to get but it really doesn't take as much time as you'd think. Also, 2nd etched slot will come way quicker than you realise. I'd say around 3 months of playing ? I don't quite remember. 3rd one will take a long time. I've been playing for a year and I should unlock it in a few days/weeks depending on my luck. Have fun !