Translation:
How can I solve it?
iPad iOS 26.3 has damaged iPad.
I have this problem with my iPad Air 5gen 64Gb: previously iPad iOS 18.7.3 was installed and everything was ok. Later, 2 days ago, I installed iOS 26.3. The iPad is stuck and no longer works, it restarts continuously in a perpetual loop without touching it, sometimes briefly causing the purple screen to appear. I tried to restore it to factory values with a PC and apps like Apple devices and iTunes. The iPad has not been restored because it restarts continuously. At startup, the iPad does not activate, indicating an activation error, but it still indicates that the IOS 26.3 version has been installed. Today I went to the Apple store, but not even here they were able to start my iPad and solve the problem. Neither replacement, nor repair because out of warranty, I was offered. In conclusion, it was much better not to have updated to IOS 26.3. Did it happen to you?
You appear to have explored your available options - concluding with a visit to the Genius Bar at an Apple Store. If the Apple technicians were unable to resolve the issue, then you likely have an unrecoverable hardware fault.
Unrecoverable failure subsequent to performing a system software update is rare. Outside of warranty, if you don't have benefit of an active AppleCare+ plan, your only remaining option is to send the iPad for repair - which may be costly.
For practical purposes, in lieu of repairing and returning your own iPad, Apple usually exchange the iPad for a renewed/refurbished iPad of the same model and specification. The cost of the exchange is the same as would otherwise be charged for repair. Unless you have benefit of an active AppleCare plan, an out-of-warranty repair while expensive will be somewhat less than the cost of a replacement iPad.
As for out-of-warranty repair costs, Apple offers a fixed-price price repair. You can verify eligibility and cost for your region/country here:
iPad Repair & Service - Apple Support
If you have an active AppleCare+ plan, the cost of repair is reduced to a small excess/deductible fee.