If the App is not compatible with an updated version of the OS - and the App itself is no longer supported by the App Developer, then you have an insurmountable problem; you cannot revert to an earlier version of iOS/iPadOS. Have you tried reaching-out to the App Developer?
How to contact an app developer – Apple Support
Whilst recognising that the following comments are based upon hindsight, for critical data, it is unwise to maintain reliance upon an App that is not actively supported by the App Developer. Likewise, you are always well advised to verify compatibility of essential Apps, with an updated version of the OS, before committing to the update.