Make my own 'prep meals'

If you have the time to make a sauce from scratch, then that's almost always going to taste better than something from a jar or mix, especially if you can use fresh herbs and spices (as opposed to the dried/powdered ones you've had in the cabinet for ages). You can also meal-prep a huge variety of foods beyond just rice and chicken with a sauce. I will say, though, that that formula (a grain + a protein + a vegetable + a sauce) is not a bad basic framework for meal prep, and you can just about roll some dice or throw some darts to pick one of each of those and come up with something tasty no matter what.

Whatever sauces you like!

Here's the Buff Dudes' Beginner's Meal Prep Guide

https://www.youtube.com/watch?v=YOPs3zUIMwU&t=533s

Prepared/jarred sauces are fine, if they fit your calorie/nutrition needs. But it's pretty simple to make some good ones on your own, have more control over flavor and calories.

If you like a little heat, try gochujang or other chili sauce, peanut butter powder (defatted or semi-defatted peanut flour), rice wine vinegar. To that, if you like, you can optionally add one or more of minced garlic, chopped scallions, a few drops per serving of toasted sesame oil if you can spare the calories, or any other similar thing you enjoy.

Many foods do nicely with a mustard vinaigrette, which is just prepared mustard (flavored ones are fine, if you like), plus decent vinegar (red wine or white wine vinegar are fine, plain old white vinegar is a little flat for the purpose). Normally, vinaigrette would have some oil (like olive or avocado oil), but in most cases I prefer to leave that out, add some fats in the form of toasted pumpkin or sunflower seeds, or nuts, for more flavor. Add anything else you like: Garlic, onions, peppercorns, capers, herbs . . . .

Miso (fermented grains/beans) is also a nice base for a sauce or marinade. There are different types, from hearty brown (hacho) to moderate red to sweeter white (shiro). You can thin with broth, vinegar, or just warm water depending on what you're going for; and add any other seasonings that sound tasty. If you can find live-culture miso, that's a plus for probiotics. But even others are tasty.

I don't know where you live, but here in the US there's a type of broth base (paste form, in a jar) called "Better Than Bouillon" that comes in a wide range of flavors, from various meat bases to veggies, mushrooms, or garlic. There are lower-sodium types, but even those are salty. However, it's IMO more flavorful/tastier than bouillon cubes or crystals, and keeps a very long time if refrigerated. There's a lot of flavor for few calories.