Do you have Max volume enabled ? Disable max volume and you can then control the variable volume of the box with the remote.
I typically keep the TV volume at about 60% or 70%, and then use the variable volume on the box.
Also, the BT250 has 4 button (on the top surrounded by blue line), which are IR learning buttons. So you can even have the remote memorize some of the buttons from your TV (but keep in mind that is IR only, not bluetooth obviously). So those 4 buttons can mimic any 4 buttons of your TV. most people will use them for power, input select, and TV volume +/-, but you can really use them for anything you want. Since in my case, I don't use any other input, I had mapped the "input" button to be the "sleep" button from my TV remote.
If you want to program the 4 buttons, you can check out the instructions in the manual: https://buzztv.com/wp-content/uploads/2023/03/BT-250_Manual_V1.1.pdf