all third party apps on iPhone SE not opening, but all stock apps are.

I have no issues opening third-party apps and, like you, I am a public beta tester. go to the feedback assistant and report this in the thirdparty apps area.

Hi,
If I were you, I'd put the phone in recovery mode, and restore it.
Yes, its time consuming, but I had to do this with my 12 pro max last night, to fix an issue I was having, where voiceover wouldn't read back dictated text, after I had finished, and it worked.

It also allowed me to have a cleaner home screen, and fix my issue with braille screen input crashing, which was intirely my fault, because I was unknowingly turning my fingers, which then moved the rotor, when I performed the, "delete last typed word" gesture, which is a 2-finger swipe left.
I'm using braille screen input now, to type this comment, and its working flawlessly.
It hasn't crashed once. It actually did crash once last night, right after I restored, but that's because I moved the rotor, while trying to delete the last word I'd typed. I was able to verify that the rotor was being changed, because I was on the phone with Apple accessibility, and they were screen sharing with me, so I could show them what was happening.
So, I went into braille screen input, and typed something, intentionally making mistakes, so I could show him what was happening, and he said, "yup, you're moving the rotor when you delete by word, so try to keep your fingers straight." and it helped.