How to downgrade my iPhone to an old iOS?

Lots of scurrilous accusations about Apple going on here. Sorry, there is no way to revert to the older iOS version. You either start troubleshooting or take it to an Apple Store or authorized Apple service provider for help.

Have you tried turning the phone off and restarting it?

Force restart iPhone - Apple Support

Or you follow through with your threat to change companies and hope you don't have the same issues. No one here is an Apple employee or Apple contractor, just fellow users who volunteer to try and help.

You will very likely find yourself being just as frustrated if you have a device from another brand that launched 5 years before the latest update and notice the typical issues of the device slowing down, having battery issues, etc. It can and often does happen when you install major updates on older devices (not just phones) that is many years removed from when that device first came out and was at the time the most powerful options that had been released. You should also be aware that it’s possible that your 11 Pro may be nearing the end of its support for new major updates…we won’t know for sure until the next major update is announced but it’s very possible that the 11 series and older devices that were still supported on iOS 18 may not be supported on iOS 19 or whatever that update name may be. Some brands don’t support their devices with major OS updates for 5 years…although just like with the iPhone the devices can still usually function without access to the latest updates until it becomes an issue where app developers stop supporting older iOS/OS.

As far as the battery situation…you haven’t mentioned if you have ever had the battery replaced on your iPhone 11 Pro…if you haven’t and you’re still using the original battery it’s probably been time to replace it (5 years is extremely impressive). You also haven’t actually said what troubleshooting if any you have done for the issues you are having other than just pointing out that the update is at fault. Unless your iPhone is completely broken and can’t function at all it’s possible that some things can be done to try and help. It would just take being a little more detailed in what has been going on and what steps you have tried to address the issues.