Internal TestFlight testers have to be App Store Connect team members with a role, but none of the roles fits someone whose only job is installing and testing builds. A QA tester ends up with Marketing, Customer Support, Sales or Developer, and each one gives access they don't need: reports, replying to reviews, or managing builds and certificates.
External testing avoids that, but every build then goes through Beta App Review first. For a small team testing daily builds, that delay defeats the point.
A "Tester" role would fix it: internal TestFlight access to the selected apps, and nothing else (no metadata, reports, reviews, pricing or certificates).
I've filed this as FB25053990. If you want this too, please file your own feedback and mention that number. Duplicates are how Apple gauges demand.