Security from a from a Mustachian perspective is two parts mental/emotional, one practical. I view you as able to FIRE at will from a practical viewpoint, but at risk from the mental/emotional side. The sides meet at the altar. I suspect that you have a 49% chance of getting everything you want just fine if you quit today, and a 51% chance a year from now. The outcome will depend largely on your emotions and decisions. Further earning will have only a secondary effect. Roughly: If you're determined to quit, quit now. More work isn't going to save you.
If your goal is to be secure, focus on mental/emotional upgrades and thrift upgrades, with special focus on building relationships built on a thriftier approach.
One part of Mustache security is from having the ability to live happily on a small amount. That's unproven in you, and not your habit. Best summary: you might be capable of it, but it's not in your current wheelhouse. Therefore FIRE will always have an element of incompleteness (you'd like fancier stuff!). You can strengthen this through practice if you like (maybe read the MMM post on Stoicism, experiment with such techniques), but unless you do actually practice joyful thrift in some way, your happiness and financial security will always be a little precarious.
Another part is the ability to note and control spending with clarity and decisiveness. You are at a C level with this - you have some data, and do make some control decisions, but it's unclear whether this is at a level that will reliably protect you for 40+ years. It wouldn't surprise me if you overspend at some point, then have to reduce spending later.
My guess is that marriage will be a bigger test of this than you want think, with a good chance of forcing you into overspending and future reduced spending. Your description of girlfriend doesn't sound like a person who supports thrift, but rather a person who wants to grab your lifeline of pleasant spending. It's hard to be thrifty for two people when you want to spend and so does she. What if she becomes unable to work after a year or two, but still wants a glitzy life?
That said, you are making plans that sounds possibly feasible and proposing guardrails that might be sufficient. It really depends on future behavior by you and your girlfriend/fiancee. I trust current behavior more than future behavior, hence my skepticism. Sorry if I sound mean, but honesty helps you more here than politeness. At minimum, can you ensure that she pays for a fairly fat Long Term Care Insurance policy? Does your proposed prenup make her responsible for that?
If your current spending is 68k or below (1.7 million x 4%), I'd feel a bit safer; if 50k or below, much safer, though not fully safe unless the case includes girlfriend's spending. Do you know her spending?
If your current spend over $100k, I wouldn't feel safe unless you presented a much more precise case comparing current spending and future projected spending. Care to post one?