In Affinity Photo you can use the alt key with the brush tool to temporarily bring up the color picker. There needs to be a similar hotkey that can bring up the color picker while using the flood fill tool.
I’m a professional cartoonists, and laying flat colors underneath a layer of black line art is a crucial part of my job. The way that the Flood Fill tool and the Freehand selection tool preform will have a direct impact on weather comic artists buy your program or not. Please consider sitting down with some comic artists and hearing their needs. We want to buy this program.