If you are satisfied with your RSB30 satellite, buy another one like it. If you need LAN ports or something, read the product descriptions of the other two models carefully. I bought my parents a second RSB30 so they could have good wifi reception throughout the house.
According to the digitec description: It also works without problems with the existing devices from the service provider, so that you can benefit from better WLAN everywhere.
I have not "connected" an additional range extender, but I can imagine that this works without problems, with the restriction that you do not have a mesh function on the external extender. The Orbi RBK 30 comes with additional satellites.....