If you'd like to see an option to withhold exact taxes automatically (to avoid owing extra with your tax return or getting a refund in the following year), there's an item on the public roadmap that you can upvote.

FWIW though, I expect implementing exact withholding may be a bit more complicated than you'd imagine because of how complex the interactions with the cash-flow priority system can get. To know how your tax brackets align with a particular income source, you need the complete picture of everything else that happens in that year, including every applicable deduction, contributions to any tax-advantaged accounts, any dividends taxable as ordinary income, etc. That's why it is currently assessed at the end of the year (like how it typically works in the US), when all of that information is known and finalized.

My intuition is that it may be difficult for the sim to know everything that's going to happen ahead of time when you consider that cash-flow priorities are often set up to operate on excess cash-flow, rather than predefined amounts per year... so those cash-flow priorities need to know how much is left over to work with, but whatever you contribute to things like pretax retirement accounts would actually end up affecting your taxable income, and thus also affecting what is left over after exact withholding, and around and around that cycle goes... Unless I'm missing something, it seems challenging to ensure that all possible cash-flow priority configurations would be solvable with that approach.

That being said, I agree exact withholding would be a really nice switch to be able to toggle on and off. I just haven't quite wrapped my head around the best way to go about it yet.