I know exactly what you mean, Amanda!!! I struggled with this for a long time as well and I still struggle from time to time but here is what I found helpful.
I consider my audience. Is this someone who is asking because they really want to learn more and want to really know how to help my child or is it someone I am telling so that my child is protected? Is it someone I am having a casual conversation with (maybe they asked why your LO isn't eating food X like their little Joey, etc)? Then I use different responses for different people.
If I need a short response for someone that might be around my LO when she is eating (maybe a play group mom we see every week, or a non-close family member?), I generally say that my daughter has FPIES, a food allergy, and reacts to multiple foods (if it is someone that is going to be in a care-taking role, I focus on her safe foods and not as much on her triggers since we still have quite a few unknowns) and that her reactions can be complex due to them being delayed from the time she actually ate the food, due to the fact that she has gone into shock from reactions before and could require ER room care, and due to the fact that a reaction can occur from trace amounts of a food (like a contaminated pan).
If it is a casual conversation, I say that she has FPIES, which for her means multiple severe food allergies that cause delayed reactions. If it is necessary to qualify severity, I say that the delayed reactions can lead to shock and have previously.
If it is someone who is really looking for more in depth information, I give them a little bit more of the background but kind of translate things into "everyday" language. I give a lot of examples; for instance if I am telling someone how it is different from "typical" IgEs, I say "If I gave her a bowl of rice right now, you are not likely going to see hives, swelling, or any kind of more immediate reaction. She will appear fine. But 2 or more hours later, she will become very sick with projectile vomiting (describe symptoms further here if needed) and might need to visit the ER." I go into more detail based on what they are interested in learning about--- some people want to know why a test can't tell us what foods are safe, what the deal is with outgrowing her triggers (wouldn't we all love more answers to this one!!!!!), etc. And I just go from there. When I do get into a more lengthy conversation than I am comfortable with, I try to end things by saying something like "You know, I know she has a lot of restrictions and it can be difficult dealing with FPIES, but she still just picks her nose one finger at a time" And then I say something about B's non FPIES attributes/interests. I worry about the lengthy conversations making people feel like B is defined by her FPIES or that it makes her an anomaly. I want people to respect us and help to keep her safe, but I also want them to realize that this is just a part of her and that I do not want it to exclusively define her. Also, little people have big ears, and since B is my partner in crime and typically by my side, I try to temper things for her.
Not sure if this helps, but I hope it gets you thinking more of what will work best for you guys. I know I always like hearing what others do so that I can get thinking about what to do to best fit my kiddo.
Sometimes I say things like "She has a severe immune system disease"...and then wait for their concerned look to say "Dont worry she isnt contagious." Or just starting with "She is allergic to all food" usually gets some fun reactions. Kind of hard to blow off those. Hehe. OK OK.....people who knows us get the true education on FPIES. But those random strangers that try to feed my kid? They get the other responses.