iPhone 8 acting weird after updating it to iOS 14.4

I have an iPhone 8 (Battery replaced) which was working totally fine few days ago. After I updated the iOS to 14.4 it started lagging and acting weird.

Now the major issues I’m facing are:

• Phone restarts automatically
• Battery percentage doesn’t change whole day. It gets stuck on 100%. I need to restart it to get the updated status.
• Phone hangs whenever I put it on charge
• Touch stops working also sometimes
• Not being able to make and receive calls sometimes

In the analytics section, it showed errors like ‘Panic Full’ & ‘Thermal monitor d’

I gave it to a local repair guy. He spent almost more than a week on repair but no luck. Can anybody help me identifying what exactly the issue can be?

Thank you.