Am I missing something 
If you want to switch from beta to the stable channel, then please uninstall the Avast Secure Browser Beta version and install the latest stable version from the official website: https://www.avast.com/secure-browser
Keep in mind that if you use Avast Antivirus Beta, you always get Avast Secure Browser Beta.
Interesting, seems strange if you have to effectively uninstall everything, avast first (presumably, to break that linkage) and then the Beta ASB.
I'm not sure I like that idea much as you could temporarily be without protection (even even if for a short time) unless you do things in the right order.
Download and save the avast off-line installation file for regular release version of avast premier and the uninstall utility.
Do a clean install of Avast Premier.
Uninstall ASB beta, reboot.
Now you should be unlinked and be able to download the regular release of ASB and install.
This assumes that you had the beta version of avast premier installed, if not then Avast need to do some testing on the removal of ASB beta.