Can I keep my Google Sheet and still get an app?
Yes. Connect your Google account, point at the sheet, and PaprPlain reads it and builds the workspace from it — and it will read it again any time you ask, so a sheet that something else keeps writing to stays useful. You can also just upload the file and be done with it, which is what most people end up doing once they see everything working in one place. Either way it only ever sees the files you picked, rather than asking for the run of your Drive, and if you disconnect it later everything already brought across stays exactly where it is.
Only the files you pick
You ask, and it fetches what has changed. Excel and CSV work the same way. Or describe the business if the records are still in a book.