The following devices - iPad maxed at 16, Macbook Air maxed at Mojave, iMac maxed at Ventura, and iPhone 13 mini on 17. Soon to add newest iPad Pro.
Way back in the increasingly dim recesses of my mind is the memory of a mention that some iCloud feature of newer iOS would not sync to older OSes. That’s main reason iPhone is still on 17.
Here’s what is important that I sync to iCloud and be able to access on all the above devices - mail, passwords, keychain, notes, calendar, contacts, safari, wallet, books, pages, numbers. And Photos - I do not use Photos app except to use iPhone camera (later transferred to iMac Lightroom) and to keep some assorted photos synced to all devices.
To date, everything syncs flawlessly. The Air is used daily primarily for Quicken (offline - I don’t download from banks), long emails, word/document processing, iMazing backups, occasional online work.The iMac is primarily for Adobe Lightroom Classic offline (not cloud version) and other photo work. I am online mostly with iPad and iPhone. The longish-range plan is for the iMac to outlive the Air and assume its more daily tasks.
My question arises because I am about to retire a VERY old, cranky and limping iPad Air (which some say resembles me 
) maxed at iOS 12, used for bedtime internet and reading, and pass its duties on to the iPad on iOS 16. Then get a new iPad Pro which will come with newest iOS.
The new iOS is intimidating enough without the worry about possible sync issues. Advice? Suggestions? Warnings? Reassurances? Dad jokes?
Thanks, as always!