I honestly find it easier to project in terms of actual planned transactions, rather than at the category level. For me, I’ve found it conceptually easier to enter in all the future planned income and expenses as manual transactions, and then use formulas to calculate the future account balances as far into the future as I want. Editing the manual planned transactions makes it really easy to plan almost any kind of hypothetical spending or saving scenario.
Sadly, even though the concept is simple, the implementation isn’t quite so. I’ve currently got some really hairy formulas to do the future balance calculations, and you have to manually reconcile the planned transactions to the real ones when they arrive. @jfederline described a similar approach here: Add a Projected Balances sheet
This approach would save you from having to manually keep all those future budget categories in sync all the time. You’ll have to decide if this way is any easier, but hope it helps.
FWIW, I’m working on a Google Sheets add-on that would automate this (including automatic reconciliation and future projections of scheduled transactions). I’ll share it here when it’s done.