Hi Nick - congrats on the decision to begin this journey! Should probably be in the "ask a mustachian" section, but whatever. The topics are around here are reasonably flexible, so don't fret it - the most important thing is that you decided to post!
1. One thing financially you may want to consider is an emergency fund. Some people keep about 3 months spending in cash, others keep 6-8 months, and some people don't believe in having one at all - instead opting to use 'springy debt' and monthly cash flow to cover unexpected expenses. Either way, consider whether you need an E-fund and think about a number you'd be comfortable with - there's not really a right or wrong answer. Factors worth mulling over include whether you own vs rent, have kids, job security, risk tolerance... things like that.
2. I think most around here would agree that you should keep the 0% debt for now and save/invest the money instead. Your call though. In the big scheme, $20k is a small drop in the bucket if you really adopt this FI mindset.
3. The most popular method is to pay yourself first. On payday, transfer money to investments immediately then use the remainder to take care of living expenses. Although, once you get used to investing for a little while saving will become more meaningful than frivolous spending. If the FI thing really "clicks" this problem will eventually just go away & you'll begin to spend based on your own values then automatically invest the giant surplus every month.
4. Decide on one and stick with it. Most around here advocate index investing with a heavy equity allocation. Once you make your decision, write out an investment policy statement. Doesn't have to be anything really fancy - just a one pager with your target allocation, rebalancing discipline, etc.
One other comment - the 55 number is way too conservative IMO. Even if you don't retire, you may want to consider significantly increasing your FI goal. 55 is almost 30yrs away for you. Anyway, I think once you get in the swing of things you'll see that FI can happen way, way sooner for you.
Good luck and welcome! Glad you are here.