I'd just add that if you're paid fairly well now, a good strategy to get that extra oomph would be to get a low paying but easy (and possibly fun) job to cover the gap. I'm not as much of a tax expert as MDM and the others, but it does seem to me that having $10K to $20K a year in earned income is treated really well by the tax code - you might get EITC, you'll probably get some retirement savings contribution credit, you'll probably get good ACA subsidies, and there may be others.
I've been FIREd for about 5 years and although I don't need the income and my Roth ladder is overfunded, I still end up doing Roth conversions each year because I have nonrefundable credits that go to waste otherwise. A part time job would also sop up those credits for me as well.
You also might find that your spending is lower than you are currently predicting. I found out that some of my spending right before I retired was stress spending - going to Burger King because it is salty, sugary, fatty, easy deliciousness turned out to be stress relief valve of sorts. Similarly vacations - I've been a lot of places already, and just hanging around my house and not working is pretty nice - no need to fly somewhere else for a week. And if I do go I can travel hack in a number of ways to make it very cheap.
To be clear, I can afford both vacations and Burger King whenever I want. I just don't feel any pressing urge for those things anymore because I don't have the stress of working.
Did you include your Roth contributions in your Roth ladder funding? Those come out first and are penalty- and tax-free, so arguably should be considered part of your ladder.
Between a part time job, lower spending, Roth contributions, and knowing about the December to January rule, I think you might be better off than you think.