Budget worksheet: summary of various transactions

SFredrikson 

May I challenge your design, before you get too far into using it?

I have been working on my own budget worksheet (actually more of an income and expense tracker) recently, and would commend to you the basic design approach I've used.  I've had it going since 2019, all the way through last year, and will be continuing in 2024.

There are only three sheets in the entire workbook.

1. Transaction History: this contains every transaction, income and expense, and it's where I add to each line the various budget categories and sub-categories. All transactions going back to 2019 are in this single sheet. All checking accounts and all credit cards. All in one ongoing transactional database.
2. Summary: A single Pivot Table. I can filter it (and generally do) so it's only showing one year at a time, but it shows every category and subcategory by month. Perhaps because of the way I'm doing this, I don't have the problem you refer to. If in any given month, a sub-category isn't used, it isn't used; no error message, just a blank area. If a sub-category that was used in 2020 isn't used in 2021, then it appears in 2020 but doesn't even appear in 2021.
3. CatTables: this is where I have my tables of budget categories and sub-categories, which are used as the source for DataValidation back in the Transaction History sheet.

My basic point: by having separate sheets for each month, you may be doing more of the work yourself--separating things  by month--and failing in that way to let Excel's pivot table tool do the work of that separation and reporting. When we keep records on paper, it makes sense to create that kind of order, but in moving things to Excel, it makes sense to let Excel take care of creating monthly summaries, if needed, quarterly, etc., and doing so from a single ongoing--for as long as you keep your accounts running--database.