If you don't use Braiins OS+ the pool keeps its 2% fee instead of giving it back to you.
The point of using Braiins OS+ isn't just to get a discount in Slush Pool, but to improve the miner efficiency. As i
just posted in the Braiins OS thread, a miner just claimed
22.8 J/TH with the S19 PRO 110th model (107Th @~2450W) so you can do your own math to see if this is worth or not...
And now that you mention, the A series was recently added in the latest 2022-03-21 minor release, there is no need to force the model detection anymore. Of course, the whole x19 support is still in "early access", so if you don't like to participate in public tests, you can simply wait for future releases.
The point of the Slush Pool discount when you use Braiins OS+ with it is to make it an even better deal for you. You are free to use any other firmware with the pool, or any other pool with the firmware.