iCloud sync and backup confusion

I have an iMac desktop and a new iPhone 16 Pro. I use iCloud for syncing and have been using it for backing up both devices too. But I am starting to feel really confused about these features.

I know I can choose which apps to sync and which apps to backup, but I struggle to get my head around exactly which apps NEED to be synced or backed up. Am I understanding the following points correctly?

• apps that are already cloud based such as Trello, Google Calendar, Gmail, Facebook etc - really don't need syncing OR backing up, as they are always accessible from both devices and should sync on the servers?
• The only things I need to synch to iCloud are default Apple apps, and other apps that I use on BOTH devices, so that I am always getting the "live" version regardless of which device I am on?
• Anything I only use on one device, does not need syncing to iCloud?
• Backups - it is only app data that is being backed up and system settings? Not actual apps? So if I ever need to restore from a backup, the restore process with automatically re-install those apps based on the data that has been backed up?

I am probably well and truly over thinking this, but I don't want to pay for anymore additional storage than I am already paying for, and I feel like I may well be synching and backing up things I really don't need to.

If anyone can enlighten me I would be very grateful.