For diarrhea - I use Imodium as you have done, or codeine (Panadeine) if I also have a bad headache (codiene and Imodium work on the same receptors so I don’t use them together) -I find that codeine is not good for use for more than about 3 days as I become tolerant to it and it can screw with your pain perception systems. Plain, non - fibrous food (carbs and starches, like sweet potato, potato, rice, and bread, and easy to digest protein, like fish and eggs - I avoid salads or fibrous vegetables while I have problems as they’ll exacerbate symptoms, but hit them hard when the symptoms ease to help ensure adequate nutrition). Good multivitamin. Bananas (know to be a natural anti-diarrhea food).
For nausea, Maxolon (but it’s a motility drug - so works against Imodium, and depending on the cause of the nausea may not be effective, have also could tried stemetine instead). Good organic Lemon and ginger tea.
For stomach cramps, hot or cold packs, I find the wheat ones you freeze or microwave best. I’ve also found peppermint tea beats buscopan every time (and the calming effect on your whole gastric system can be great for nausea as well). Not to be used with severe reflux or GORD - it relaxes the sphincter between your stomach and esophagus, so exacerbates reflux / GORD.
I haven’t found much for the fog, I’m afraid, except to forgive myself. In my case, caffeine makes the stomach issues significantly worse, so it is a net loss to treat the fog this way. Just extra sleep, and scheduling doing the most menial tasks the day after MTX (admin like time sheets, travel claims, or at home, laundry, dishes etc).
Learn the locations of convenient toilets in situations like long commutes, or at the start of a full day conference. Go hungry if in doubt, and eat high - quality calories when at home with easy access to toilets.
I know this sounds like a lot, and it can seem really overwhelming. But it will pass, one way or another. Either the symptoms will ease a bit, and you’ll develop strategies to deal with them, or you will be considered intolerant to MTX and they will move you on to another medication.
Remember to give yourself a break. For me, I finally went to talk through it all with a psychologist about 2 years after it all started, and found it immensely helpful. I think it would have been great to have that support at the start - sometimes it’s just good to get an outside perspective to realise you are actually doing an amazing job handling it all, even if it doesn’t feel that way - that’s certainly what I see in your posts