I just did myself the test on a separate device with Google play + an old Phoenix wallet. Had like 2000 sats in it, legacy Phoenix even. 1. Saved the seed 2. Uninstalled from Google Play. 3. Download from Github the apk and install it 4. restored with seed 5. Funds were there just fine. 6. In Google Play account do not appear anymore as installed
That must have changed then. But still, even if you keep it installed on Google Play: installing updates via sideloading should just work. The uninstall step is unnecessary.