Congratulations!
I'm late to the party, but here are my pointers (credentials: 18 year anniversary coming up).
1) People who got along great for YEARS can start fighting once married. There are two reasons for this: First, we all carry cultural baggage and expectations about what a spouse is supposed to do, and these can be different than what was expected from a girlfriend/boyfriend. Marriage can switch these expectations without us even being aware we are holding each other to a new standard. Second, the point of marriage is to make it harder to leave the relationship. This can mean a sense of security that your partner is not going to bail on you when the going gets tough, but it can also mean there's no reason to treat each other well every single day like you did before. I.e. you give them attitude and think "what are they going to do, leave?" The lack of daily incentive for kindness can also creep up on us without our realization.
2) Invest in at least a dozen sessions of couples counseling from an accredited, secular professional. The cost - if there is one - is a pittance compared to the potential value. This is where you invest your savings from not participating in the wedding-industrial complex. Even if you have nothing to fight about, a good counselor will uncover and illuminate areas where you both can work to make this transition smooth. Investing these hours and being open and honest with the counselor will send a strong signal of commitment to each other and, more importantly, will break the seal on the idea of getting outside help if things get rough in the future. So many couples spend five figure amounts and hundreds of hours getting divorced when they wouldn't spend a copay (if that) and a few dozen hours on counseling. Seriously, it's a great investment.
3) Now that you're committed, come together with a tentative life plan. Maybe it's FIRE. Maybe it's a life of helping others. Maybe it's seeing the world together. Maybe it's kids and family. The issue is a lot of people spend years in a relationship with the false expectation that their partner shares their life plan, only to find they don't. Be explicit about what you think is valuable in life, and talk this stuff out. Once you have a shared path, acknowledge how no plan survives its first encounter with the enemy! Agree not to be frustrated when the setbacks are encountered, and maybe consider some backup plans that are acceptable.
4) Don't go too fast, but don't go too slow either. Too fast would be assuming your partner is OK with plans or dramatic behavior shifts that you haven't talked through. You simply can't run as fast holding hands with someone else. Expect to have conversations before jumping on career opportunities, making purchases, starting new habits, rearranging things, etc. Too slow would be spending years waiting until the stars align to make big moves, like having kids, moving to a new place, going back to school, etc. The stars never align, but opportunities are lost every single minute. Another way to be too slow is having life objectives that are so far away (e.g. FIRE) that one or both of you frustrated by the lack of immediate rewards, feedback, or visible progress, and this translates to a feeling that the relationship is not achieving its goals. A good mix of short-term and long-term goals can help with this.
