I think that two-path approach can work fine. I’m not sure I can answer your question about how long a large JSON data set will take to download. But JSON is just text so it shouldn’t take up much room.
This site has sample large JSON files you can download: Exploring the power of JSON: a real-life JSON file example collection
So you might temporarily replace your Firebase path with data from there and test the speed of it in your Thunkable App. Just make sure to backup any existing Firebase data before you do that.