That is the reason why I decided to put my portfolio on DeviantArt, where there's bound to be artists that can cover for my poor graphical skills.
I asked this same question to friends on Discord, and one of them agrees with you in that I should replace my commissioning model entirely. Like you're saying here, one price for the full game based on how long it'll take. I believe your advice is good.
You can see my achievements in the following sites: (just ignore the 3D Custom Girl foot fetish pictures in the rest of my DeviantArt profile...)
https://www.deviantart.com/leotheevil/gallery/94887856/game-making-practice
https://fourelements.newgrounds.com/games