OP here. I have a S9+ which is not locked to any network, using BTU CSC & stock firmware. I use a 3UK sim.
To answer questions and points raised in this thread so far, if you can't yet register for the beta, then first thing it to make sure your Samsung Members app is the latest version.
If it is up to date but the beta notice isn't there then either wait for it to appear or visit XDA forum if you want to mess about with the inner workings of your phone, and possibly void your warranty or risk bricking your phone if you mess something up.
The beta wont be availabe to everyone.
If you do manage to get the beta, you are effectively testing the software, and should participate in the beta feedback and bug reporting forum. You don't see this forum unless you are a beta tester. It's an exclusive clubhouse, but no red carpet, champagne, or canapes.
Also yes the battery drain is an issue with the current version of the beta.