Hi Kevin - great work, really impressive to see how far you've come. And congrats on getting engaged. I have some thoughts about your question related to helping your fiance pay off her debt.
So, I got married fairly recently (2 years ago), and I think that one of the best things that we did was start to slowly merge our finances during our engagement, and discuss budgeting, etc. So maybe you guys should consider something similar, which would help you figure out a plan of attack for her debt as well as some of your longer term goals.
We started just by tracking our expenses for a few months (something you already appear to do - does she?). Then we made a joint monthly budget that took into account both our shared expenses like rent, food, utilities, as well as individual bills and fun money for hobbies. It also included some short term savings goals. We then created a joint bank account. We decided that to each contribute equal amounts to the shared bank account (we were both in grad school and making similar amounts at that point, but you may consider doing it based on percent of income). We used that account to pay all of our shared expenses, as well as put money away for an emergency fund and some other things we were saving for (like the wedding). This allowed us to get used to thinking about money decisions together, while still retaining mostly separate finances (our pay checks were deposited into our own accounts, and we could spend that however we liked). We used YNAB to budget and track spending, including personal (non-shared) spending. This encouraged us to be open about our spending, discuss our priorities, our weak points, etc, while still keeping most of our income (aside from joint expenses) separate. Luckily, we had no debt, but my husband had never really budgeted before, and needed to get a handle on his impulse purchases (which has has done very well), and in general we were just spending too much. We trimmed a lot of fat, and have reduced our average monthly spending by about 1500$, which was great.
After we were married, we merged a bit more. We started having all of our paychecks deposited straight to the shared account, and then a certain amount of money (for us it is $300 each) was shuttled each month towards our personal accounts to use as we saw fit. We use this money on hobbies, clothes, hair cuts, individual travel (sometimes i go visit friends with out him), and presents for each other. Because we are now good at budgeting, our personal + shared expenses are far less than our income. So then we started making longer term savings goals, and discussing how best to handle retirement savings, tax planning, etc. We are also saving for a house. At this point I make more than 3 times what my husband makes (he's still in school, I'm out of it). We decided to keep our spending at the level it was in grad school, and save all of my extra income for a down payment. The plan is that, when he graduates and gets a job next summer, we will have enough to put a down payment on a house where ever we end up.
So, that was really long, but my basic point is that starting to merge your finances now in a slow, step-by-step fashion will facilitate conversations about short and long term goals, and may help you guys both decide how best to tackle her debt. Maybe what works for you both is that you take on a greater proportion of the shared expenses, allowing her to hammer her debt to eliminate it quickly. Maybe what works best is for you to be saving your extra cash towards a downpayment and an emergency fund while she handles the debt. There are many possible right answers, and she should be part of the conversation in determining a solution.
Best of luck.