If you get your bonus in December 2020, I assume you want to have your budgets adjusted for 2021 to account for the bonus?
It would seem dangerous to adjust budgets prior to December 2020. You can’t spend that money until you have it. Also, if you didn’t get the bonus, that would cause problems. For 2020, you might be using 2019’s bonus.
For the bonus in December, you could divide it by 12 and include 12th of the income in Dec 2020 and then for the next 11 months.
When you get the bonus, you could divide the fully payment by 12. You could categorized 1/12th of the bonus as Bonus Income and budget 1/12th of the amount in December. The other 11/12th of the Bonus could be Categorized as Bonus Income Deferred. Make this an Income Category but hide it.
Then every month, 2 two manual transactions. The amount would be 1/12th of your bonus. One transactions increases your Bonus Income category. The other Transaction reduces your Bonus Income Deferred Category.
By December 2021, you won’t have any money in the Bonus Income Deferred Category. And the Bonus Income category will have created 1/12th of you bonus as Income (available to Budget) every month.
If you need to spend your bonus in non-equal monthly portions, you can make those 2 manual transactions at different times and for different amounts. They just need to equal out. Also, you can’t add to Bonus Income once your Bonus Income Deferred Category becomes empty.
Would that work?
Maybe some of the accountants on here could provide additional feedback?