As a long range goal quitting refined sugar is a good thing. The caveat to this is that immediately after quitting alcohol you can experience a hypoglycemic reaction which means you blood sugar levels are quite low. This can exacerbate withdrawal symptoms like tremors, sweating, anxiety, etc.
Diet drinks are not a good idea because they actually will trigger an insulin release and that causes a reduced blood glucose level which then causes a greater craving for anything that will raise those levels; carbs, sweets, alcohol.
Check with a nutritionist on this to adapt a plan to your own specific metabolism as we are all unique and react to things differently.