I literally just finished doing the same thing for my own foss app (and I also used Flutter). Right now I’m waiting for my production review. To get people added to your closed test, you have to either collect a list of email address, or create a google group they can join. If you use a google group, that will be a bit easier for testers to join since people can click the group link, and then the test link, instead of waiting for you to add their email.
If you can’t initially get all the testers you need, I would recommend making a post on the AndroidClosedTesting reddit; I found them really helpful to get the last few testers I needed.
How was your experience with the Starlite 5? I’ve been looking for a Linux tablet recently and came across this device. I’m just wondering how well it works compared to a normal tablet. I’ve heard it’s underpowered compared to most tablets that price.