Better than!
It gives you the updated files of the next version without the need of redownloading. It is better to use it because instead of downloading ~30GB, you download at worst 2GB for only the updater.
And by "new card creator content" you mean like the sideloader modpacks do, right? You update these via KKManager every 2-3 weeks. Feel free to opt-out of the studio modpacks if you're not planning on using it!
Not really, you download the latest version and that's that. Updating is for those who already have the game and need to install the next update without redownloading the entire pack.
Progress and cards are in the UserData folder, which you should always backup.
RX 5.1 changelog states it was already unsupported because of incompatibilities. I've no idea myself because i never used it but well, hopefully somebody more knowledgable on the plugin can elaborate.