Device Failure Caused by Official iOS 18.6 Update - iPad Air 5

First, perhaps understand the Apple Support Community is a user-to-user technical forum. Contributors here are all end-users, just like you. Other than the site Moderators, Apple neither monitor nor participate here. As such, if you hope to reach (or your complaint to be seen or acted upon by) Apple via these forums, you will be disappointed.

Apple invite submission of comments via its Product Feedback portal. For iPad and iPadOS:

Feedback - iPad - Apple

Returning to your failure, all system software updates carry a very small (but not zero) risk of something going wrong with the update process - potentially with hardware or battery/power failure occurring while at a critical point of the update process.

It is indeed unfortunate that you chose to discontinue your AppleCare+ plan - as, had you maintained the plan, the iPad would have been covered for replacement or repair. Without benefit of a current warranty or AppleCare+ plan, you have limited options.

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

Had you maintained your AppleCare+ plan, the cost of repair is reduced to a small excess/deductible fee.