@Harters - First, I LOVE YOUR POST TITLE! Indeed, it is.
I was just going to commiserate on that front in case you were only here to vent about it, but then you came back and said you’re considering suggestions, so I will put my thinking cap on (and draw on all the PITA family meals into which I have had to incorporate both real and ridiculous restrictions).
Gluten free is easy if the person eats other things. I’d include rice, potatoes, and bread to cover them and the rice-heater - carbs are easy and inexpensive.
Sounds like chicken or fish would work as your central item. I do like everyone’s suggestion of roast chicken (or pieces), but a side of salmon is as easy (if they will eat it). Or, and this is a good one for prepping beforehand and finishing easily, fish en papillote with whatever flavor profile you choose.
I know you enjoy Indian food, which could actually work with all the restrictions you mentioned if you don’t make it too saucy - you could also use a tandoori (or other) rub and roast the chicken or fish (or again, en papillote for the fish).
Roasted “gobhi” - toss spices with the cauliflower and oil - is a go-to for me because I love roasted cauliflower more than any other prep. Roast potatoes without any spices are a good crowd-pleaser, though I do love scalloped, which someone already suggested, and those also can be done ahead of time and easily reheated. A simple pulao (rice with peas for eg) would work for everyone except the rice-hater, and store-bought naan or paratha or even dinner rolls should work for that person.
It’s nice that you and your BIL get along well and can share your birthdays as a group celebration. A bit of a pain to think through, but once the menu is done you’ll have a happy gathering I hope, which has become so much more valuable to us all after these past years.
ETA: What about a middle eastern spread? Could be half homemade and half store-bought. Pita & rice (which could be more interesting - say mujadara) to cover both ends of the carb spectrum, roasted chicken or fish or even falafel if you decide to go that route, a few dips and sauces from the store, and roasted veg to round it out.