I am so annoyed I can't really share screenshots (private info, c'est la vie), but I have future projections sorted out. I leave regions of blank rows in the transaction history between sections, so I can have a copy-pasteable month of expenses ready to go at all times.
This approach works because the core math heavy-lifter in this spreadsheet design is `SUMIF`, either used to get all transaction deltas <= a target date, or = it. Transactions can be spatially in any random order, and the analytical math doesn't care.