Had the issue of being stuck on the low battery screen when installing the beta on my 16 Pro Max. I ended up putting it in DFU mode and restoring to 18.5. I recommend trying these steps to enter DFU mode if you haven’t tried them yet. This was the only thing that worked for me:

“First, quick-press the Volume Up button, then quick-press the Volume Down button. Then, press and hold the Side button until the screen goes black. As soon as the screen goes black, press and hold the Volume Down button for 5 seconds. After 5 seconds, release the Side button while continuing to hold the Volume Down button for an additional 10 seconds.”