Wow this has to be the most ambitious flux core project of all time! I think a bike frame is one of the hardest welding projects you can do (thin-walled, round, thin to thick, safety critical) and flux core is one of the harder processes to control finely. This is why TIG is usually used.
People may comment that this is a bit sketchy and it might be. Be careful and keep an eye out for failures. But big respect for trying it and making it happen.
I used to make a lot of welding projects with FC and played around trying to join round tubes dreaming of one day making a bike. But never did until I got a TIG.