Sounds like you need to enable 802.1r for fast roaming between APs on both access points and then make sure the SSIDs are the same.
Also, you can improve things by splitting your 2.4ghz, 5ghz, and 6ghz (if applicable) onto separate SSIDs, with your phone connected to the 5ghz band. Since it has lower range, it will likely roam better because the signal from one to the other will be more “apparent” to your devices.
Not sure what kind of Extender it is, but if it can run OpenWRT, you can flash it to that and have more control.