You don’t have a flat stomach because you need to develop your abdominal muscle and lower your body fat. You are also eating at a caloric surplus and haven’t exercised. It’s good that you make your food at home and avoid sugar, also make sure you are limiting portions of quick burning carbs like white rice, there are charts you can look up.
To get back into cardio, just start slow. Commit to 20 mins every other day for the first week or two, jog and take walking breaks as needed. Then once you’ve built up your stamina, you can run more days in the week at a faster pace and try other things like HIIT.
Get the gym membership. It’s a lot easier to get the body you want and increase your strength when using weights instead of doing body weight stuff at home. Being around other people will motivate you. I am a big fan of the PPL program, it’s a 5-6 days per week lifting program that hits each muscle group twice. Just start off easy on the weights until your body gets used to it.