I always go to sleep on a full stomach and have never had troubles that I could directly track back to it. I also went through some periods of not going to bed on a full stomach, and noticed no difference in how I felt. So that wouldn’t be a concern if you ask me.
In the long term, having whey only vs. a solid meal probably won’t have too big an impact on your gains, provided that your nutrition is consistently on point over the rest of the day.
Personally, though, I’d make sure to have carbs after training, together with protein. Some people might disagree and there is living evidence that you don’t need carbs to get big. I also have to say that I would never try that, though.
So, my suggestion is that you either have a solid meal after training, or you add some carbs to your shake. If you’re set on not having a meal, the latter option is probably the less invasive and won’t change your habits, but could possibly make a noticeable difference for the better. Just add 50-100 g of carbs (possibly more?) in the form of powder. Dextrose would be a good choice here, IMO. Don’t think you can get better than pure glucose on the macro profile.
Lastly, I don’t think that having casein vs. whey would make a difference. But if you’re worried about digestion rate (admittedly, whey + dextrose will digest pretty quickly), you can add fats to that shake (or really just have a tbsp of peanut butter after chugging it down).