Unfortunately, your frugal purchase is unusable for your intended purpose.
Your iPad mini1 is a 32-bit architecture device - long ago obsolete - for which the are no compatible Apps available for download from the Apple App Store. App Developers and the Apple App Store have dropped all support for 32-bit Apps - most (if not all) such Apps having been withdrawn.
Available Apps from the Apple App Store are all intended for 64-bit architecture devices. The current Kindle App requires iOS/iPadOS14 or later.
Sorry, but you have acquired a doorstop.
In more detail… the iPad mini1 is only cable of update to iOS 9.3.5 (WiFi Only models) or iOS 9.3.6 (WiFi & Cellular models). Apple ended update support for the iPad mini1 in September 2016. Your iPad cannot be updated to iOS 10 or later major versions of iOS, as the internal hardware does not meet the minimum technical requirements for new versions of iOS/iPadOS. This limitation cannot be bypassed; 32-bit architecture devices are all obsolete.
If you want or need to access to current Apps, requiring more recent versions of iOS than the mini1 is capable, you’ll need to replace the iPad with a newer or current model.
Here is a list of the highest version of iOS supported by the older models of iPad (for simplicity - “generations” are shortened to a number) that are restricted in their ability to be updated:
iPad1 - iOS 5.1.1
iPad2, iPad3 and iPad mini1 - iOS 9.3.5 (WiFi Only models), iOS 9.3.6 (WiFi & Cellular)
iPad4 - iOS 10.3.3 (WiFi Only), iOS 10.3.4 (WiFi & Cellular)
iPad Air1, iPad mini2 and iPad mini3 - iOS 12.5.5 *
(*) these models are 64-bit devices
