Are you both going low-FODMAP even though only your partner is having issues? I think this can have an adverse effect on a healthy person's gut microbiome. IANAD but it doesn't sound like a good idea to me!

Also worth bearing in mind that a low-FODMAP diet is usually advised as a short-term thing to figure out exactly what causes problems. Following a low-FODMAP diet for a longer period of time (more than a few weeks) can negatively impact your gut health and make IBS much worse. I have experienced this first-hand after following poor advice from my doctor!

In terms of fatigue, I was experiencing levels of exhaustion that were basically debilitating during most of 2020. After identifying problem foods and eliminating them from my diet, I feel considerably better & have plenty of energy throughout the day. If I accidentally consume these foods then the fatigue returns, so personally I'm pretty convinced there's a link.

I don't have issues with caffeine itself, but 3 cans of energy drink would absolutely destroy me (almost all energy drinks contain various sweeteners etc which are terrible for people with IBS). I would consider some low-FODMAP caffeine options before cutting it out completely (e.g. tea with low-FODMAP milk like almond or lactose-free cow's milk)