that's not really Bip though, only original Bip, Bip S and Bip Lite have transreflective displays with battery life in 4+ weeks and display readable in sunlight

that's not really Bip though, only original Bip, Bip S and Bip Lite have transreflective displays with battery life in 4+ weeks and display readable in sunlight, any other Bip is just regular power hungry watch. Bip U has already just regular TFT display with poor battery life, maybe a better readability than OLED, but not comparable with transreflective displays and their battery life. As you said you can use Gadgetbridge if you don't trust them, though I find GB experience very poor always losing connection compared to their own app, not sure if you can disable internet connection for the app to keep using it which would resolve privacy issues (I use also GBoard without internet access).