Other than protein powder for when I want to hit my macros for the day but I'm feeling lazy, I don't use any supplements, not even vitamins. Macros (i.e. eating according to your body's daily requirements for protein, carbs, and fat, which is found using a simple & free calculator) is a whole separate discussion we can have if you want to dive deeper into properly feeding your body, but I would say it's not essential for managing reactive hypoglycemia (but is very useful!).
Pretty much, aside from actively tracking your blood sugar (my reactive hypoglycemia is not diabetic-related), you just have to learn how to listen to your body, which takes time & experimentation. The goal is simple: feel good from the time you wake up to the time you go to sleep. If you wake up feeling like crap, if you have dips during the day, if you wake up at night, then you need to adjust your diet in terms of what you eat, how often you eat, when you eat, and how much you eat.
Once you master all of that stuff, then you can pretty much cruise around the whole day feeling good. I felt so bad & so mediocre for so long that I didn't really know there was another way. You should absolutely wake up feeling great & then feel great all day long from sunup to sundown, and if that isn't the case, then it's time to make some adjustments!
For me, it's pretty simple - eat small meals every few hours. That's a big reason I got into food prep, just so that I could have things ready to go in order to stay feeling good all day long instead of having energy dips throughout the day & getting brain fog. Once you find out what works for you, the key is to simply setup a system that enables you to provide yourself with the food you need, when you need it, every day. Having a good system is essential to managing reactive hypo imo.