What is the best way to cook chicken if I plan on microwaving it a few days later?

Under such tight restrictions, one step that might help is brining. This would apply to chicken breasts, pork loin, or pretty much any other lean meat; darker cuts such as chicken thighs will benefit less, but there's no reason you can't brine those if you want to.

You can find a quick primer about how and why this works here; in short, brining denatures some of the proteins that "squeeze" moisture out of the meat as it cooks, which reduces moisture loss in the finished product. You don't really need anything other than salt, water, and time, though many recipes will call for additional flavoring agents.

Make sure that you don't overcook to begin with. If you can, get a thermometer and measure the meat's internal temperature. Here is a decent guide to the recommended temperatures for various types of meat; remember that you want to cook to slightly below these temperatures to allow for carryover.

From personal experience I would advise you to vary your technique and try to cook different variations on the same basic food; this guards against boredom, and makes it easier to stick to a regimen of weekly meal prep. Instead of broiling, cook your chicken on the stove, then make a basic pan sauce to serve over it, which will add moisture and flavor. If chicken thighs are on sale, get some and braise them instead (you don't need to bother with brining if you do this). This takes some time to cook, but it's very easy, and they'll reheat well for days.

I've been meal prepping with chicken for nearly a year. If you want it to be moist after re-warming it in the microwave, you have to cook it near perfect or even just a little under, so it will finish just right in the microwave. If your goal is to do this long term, forget about using a brine, thermometers or any other extra steps that take time. Meal prepping is about saving time.

If you use whole breasts, you'll want to cut them before cooking. I like to use tenderloins because it allows you to skip that step.

Roast the chicken with salt and pepper at 400F for some time. Make small adjustments until you get the time correct - everyone's oven is different. I've found 11 minutes to be the right time for tenderloins in my oven.

My favorite way to cook chicken for meal prepping is either marinating and then roasting in the oven with veggies, or more often than not, using my slow cooker. You can still make basic sauces or marinades for the crock pot that are allergen friendly and use a lot moister (aka chicken thighs) cuts of meat that will break down really well in the slow cooker. Using citrus juices or flavors will help to tenderize the meat and still give great flavor! If you have a good idea about what their allergies are I’d be happy to make other spice or marinade suggestions too. Hope this helps!