K2 Thraxis as a beginner

I’m fairly new to snowboarding and only have around 6 days on a snowboard, although plenty of experience on skis. I’ve just bought some 2019/2020 K2 Thraxis only used for roughly £100. Come to find out afterwards that they’re apparently 10/10 stiffness boots. So first question is did I make a fool of myself and overpay for these and second question is will this be hell trying to progress in such stiff boots. My thinking was all snowboard boots are softer than ski boots and I can muscle around a 120 flex. For reference I’m around 165lbs