With new EU asylum and migration regulations only coming in effect in 2026 (two years, although it is one of the most important topics in a lot of EU countries and rightwing on the rise everywhere), this is simply taking too long to do absolutely nothing locally. https://commission.europa.eu/news/setting-out-plan-put-migration-and-asylum-pact-practice-2024-06-12_en
If I remember correctly, there is only ever one generation of backup stored in iCloud. Especially in scenarios like yours with different software versions, this is not enough. My recommendation is both still keeping the previous iOS/iPadOS version as long as Apple still supports it during the transition period. And additionally to iCloud backups, do manual backups to a PC or Mac with iTunes/Finder. There you can have multiple backup generations.