I came back to buggies last year and was F5 (doing N.E.)
I did all of them and didn't bother doing Nats as I wasn't sure if I'd get in, and I didn't want to keep travelling to them to be told I couldn't race.
After doing the Regionals, I got my F3 for this year, so stand more chance of getting in to Nats, so thats what I'm doing this year.
I'd do your Regionals, get into the swing of things properly, then do Nats next year. The regionals are great meetings, nice and relaxed compared to your TC meetings.
And the people are mega friendly too. You won't regret it.