Hey, great job tackling that rewiring! I've definitely run into similar issues on older boats myself. That flickering could be a sign of a few things, maybe a bad ground, a failing sender, or just a loose connector. I'd recommend checking the sender's resistance,it should be about 240 ohms when the tank is empty and around 33 ohms when it's full. And give those wires a wiggle while you're at it, you might catch some jumps that'll give you a clue
Oh, and quick question, is your sender mounted on the top or the side of the tank? Knowing that could really help us figure out some upgrade options