I use High5 Zero electrolytes, two tablets in 1 litre of water, which i drink on my commute to work.
I also have a pinch of Maldon sea salt flakes in at least two of my coffees most days. This is an each-to-their-own thing, people tend to find the fact that I put salt in my coffee interesting, but I don't know anyone that does it.
I don't have any constipation or digestion issues because I drink large amounts if fluid every day. I have 4-8 mugs of coffee in my working day, plus 1 litres of water, twice a day in my commute. I probably drink 5 litres a day.
I bought collagen powder, but I don't really use it (my young adult children like it though!). I don't want the calories of protein powder on fast days. I figure if I'm going very low calories (I can't claim true fast because I have milk in my coffee) I don't want to drink 100ish calories of collagen powder when I could have zero calories electrolytes.
I spent a month or so having a collagen protein drink all non-fast days but honestly don't consider it made any difference. I eat clean on non-fast days and morally object to the processed nature of protein powder because it doesn't fit with my whole food approach. That said, I do have a milk-made protein shake as-and-when I feel I need it. Just not frequently.
Regarding the benefits of a longer verses shorter fast - we'll a short fast is the norm for me. I haven't had breakfast for ages. I used to beat myself up for not finding time to eat in my working day. So giving myself permission to be OMAD was freeing. I can't imagine being anything other than OMAD weekdays at any time. Longer fasting was a natural extention to this.
I'll be honest that the main benefit of fasting for me right now is weight loss. I'm on a journey from morbidly obese and part of teaching myself to listen to my body was to move away from narratives around eating because I should, to eating when my body needed fuel. I had to accept that, being overweight meant that my body has ample fuels cells to burn in my body fat, so I would not starve by not eating. That would be different for someone at a lower weight.
I take a multivitamin plus an extra Vit D tablet every morning, so im covered for the essentials. My husband (a runner 🏃♂️) has started taking magnesium every morning (instead of the vitamin D I take, because he runs outdoors so gets ample VitD). Im considering adding his magnesium to my morning vitamins.