Hi mate, I'm from a League background but switched to Union when I was 13 ish and switched towns.
Depending on how big you are, number 8 sounds like an option. They're normally the best ball carriers in the team. Flanker is difficult for someone switching codes because they're job is to be good in and around the breakdown and that takes a while to learn.
General differences are you don't work as hard. In League you might 15 plus carries and 30 plus tackles, Union you'll do a lot less work. Another underrated one is your body angle when you carry the ball, a lot of League forwards run upright and try and bust through tackles, in Union you'll want to keep your body lower so they don't rip the ball off you or hold you up.
What's your League club?