Hi. I think what you’re looking for is the Savings Budget sheet. It would enable you to do exactly what you are describing. In fact, I use it that way with a couple of “fake accounts” for longer-term savings goals. In addition to the page above, there’s also a YouTube video introduction to the sheet here. I’d recommend watching the video to get a good feel for the logic of the sheet.
The Savings Budget has become my go-to budget sheet within Tiller. Happy to answer any questions I can about my own experience with it.