Hello @JoeJoeInc 

1. For 'After a short while the WIFI clients will drop from the R7000P and from the RE650'

Please double-check if that happens with happens when RE use a different SSID or if any Leds goes off on extender at that time.:

If it is unstable issue, you could try to troubleshoot as guide here:

https://community.tp-link.com/en/home/forum/topic/272398

2. When Range extender uses the same SSID as the main router, devices normally could roam between different APs, but it's not a 'seamless' process.  Most nowadays mobile clients  supports Wi-Fi roaming, that means they could remember the credentials and roam to a different AP when signal is not strong enough.   When device roam from router to extender, there will be a short disconnection normally within a few secs. Is this the issue you mentioned? 

If you want to build a seamless  mesh, you will need a more professional mesh system like TP-Link OneMesh(*onemesh router required) or Deco Mesh.