I have tried in the past to build prediction models and spreadsheets similar to what you've done. I failed for two different reasons. First, in an effort to be 100% accurate, the models became very complex and I was not able to accurately build such complex models - I kept finding errors in calculations, dependencies, and so forth. Second, my life and tax laws change too frequently, so the maintenance to the models was too burdensome for me to continue.
I have had more - and by more I mean some - success with some medium complexity models. These models make simplifying assumptions which I sometimes notate in the models themselves, consider reasonable to make, are likely to be neutral rather than optimistic/pessimistic, and are ones that I estimate will not degrade the output of the model beyond the already uncertain bounds based on the natural fluidity in my life due to outside influences (tax laws, my kids, my Dad, the market). I'm also able to maintain them easily enough.
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It's hard to plan when you don't know how the future is going to turn out. I've tried doing that in some cases, by assigning probabilities to various outcomes and weighting them to get some sort of expected value number or calculation. But that uncertainty - will my kid need braces / go to college / go to Harvard / drop out of college? - is really so big that I've mostly decided to make those sorts of decisions on gut feel and SWAGs rather than formal models.
It's also hard to plan for late in life and after death sort of things. I don't know what my kids' marginal brackets will be when I die. So again I SWAG things quite a bit.
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I am at around a 1% net WR. One of the things that I "buy" with that low WR is that I have very wide margins and can SWAG or gut feel a lot of these things and feel comfortable that it'll work out OK.
At some point you actually do get to decide "Hey, I have *enough*" where *enough* means you can give yourself permission to stop striving and optimizing if it's no longer giving you pleasure or satisfaction or incremental improvements that make a difference to you. I'm at and past that point. You sound like you might be approaching it.
It is more of a psychological thing, because essentially your cheese gets moved: the optimizing and spreadsheets kept on getting better and better with more tweaks and additions and analysis and ideas, and that was fun...until they don't, and it isn't. When that happens, it's natural to keep doing the analysis and work for a bit, then get a little uneasy, then maybe a little sad. Eventually, I think the healthy thing to do is to say "Good enough" and then go do something fun with your kid or start working on a different project like train for a half Ironman or paint your house or learn French.
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I have considered the impact of the fact that my drawdown isn't really a drawdown. My stash is about twice what I started with when I FIREd 7 years ago. Projecting that forward 30 years, I realize I either need to spend more or prepare my kids more for what will hit them. I'm trying to do both now, actually; more of the latter than the former.
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I currently have sort of a just-in-time pull model for my finances. I spend on my Fidelity VISA, which pulls from my checking, which I refill as needed from my taxable. But I only pull from taxable what I need. This creates a "how low can I go?" mentality, which is fun in a certain way. But I'm also starting to not want to be the richest skeleton in the graveyard or have regrets about not doing something if I want to.
I have considered a model where I do some sort of push system: Figure out what 2% of my FIRE stash is, divide by 12, and set up a taxable -> checking push of that amount. And then set a rule for myself that I have to spend whatever is in my checking over time. People who do this do have a tendency to spend it I think, and they're probably more balanced people than I currently am.
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The engaging data infographic (
https://engaging-data.com/will-money-last-retire-early) with my personal data showed that I'm almost virtually certain not to run out of money, but have a ~20% chance of being dead at 70. It did make me focus on my health more.
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There is also the notion of "the wide plateau" - there are some things that you can optimize but the effects are really small. Like if I take SS at 65 or 70, there may only be a 2% difference in expected value. That's worth optimizing a little, but not worrying about too much because my SS is only something like 15% of my FIRE stash, of which I'm only taking 1% anyway. So it really doesn't matter.
I guess the corollary there is to figure out which knobs really matter (financially or more broadly in life) and then focus your energy on those. For me those knobs are my asset allocation, my withdrawal rate, my health, and my kids. Second tier ones might be Roth conversions up to X% marginal rate and items related to a potential inheritance someday. The rest of it is mostly broad plateau stuff, at least for me.
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The book "Die With Zero" may give you some ideas also.
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HTH.
