I would recommend a pair of fishing pliers, like these:
If you need to pinch on split shot, these are handy. Also for removing hooks when they go deep, or the barb isn’t cooperating. Also if you catch pike or pickerel - you don’t want to put fingers in range of those teeth! I still have a scar from when I was 14. The pliers often have a built-in tool to cut line and even wire if you need to. A good pair can be found for under $20 if you wait for a sale. Attach a float to your new pliers. Ask me how I know…
Good filet knives are important, but you might already have that for purchased whole fish. The only other prep tool I’d suggest, and it’s a bit of a splurge, is a pair of deboning tweezers. They are fantastic for removing pin bones from filets. I use them even for Costco salmon and rockfish, which often still have bones in them.
The back-of-the-knife trick works for scaling, but wear safety glasses!