I’ll prefix my comments by admitting that I don’t have any templates in the Glide store, so I’m probably not the best one to advise. But…
If they have any more than a few rows of data to swap in - and you choose Glide Tables, then this will be quite a chore. Because as you’ve noticed, there is no easy way to get large amounts of data into Glide Tables. Even getting your dummy data out will be a manual process.
If you search the forum, you’ll find a few workarounds that people have come up with, but that’s all they are - workarounds. And nothing that you could reasonably expect a template purchaser to have to grapple with. Especially if they are expecting a plug and play experience.
As you’ve also noted, once you decide to go one way or the other there is no going back. In my view, it’s always a safer choice to start with a Google Spreadsheet, even if you don’t actually use it. At least then you are keeping your options open. I shared some of the reasons why I always do this here.
But again, I don’t put my apps through the store, so I could be way off base.