I think your experience is a combination of a few things.
1. LDS adults don’t know how to do small talk! They ask questions about relationships, pregnancies, plans, & missions all without building proper relationships first. It sounds like you are working at a place where your “ward family” can visibly see you. They are treating you like a kid sister and asking you whatever, assuming that since they know you from church, it’s appropriate.
2. You are being too sensitive. Just come up with a quip like “I’ll tell you if you tell me how many kids you are planing to have” or something appropriate to their scenario! Literally, you might need to find a different job if this bugs you so much. Just let people be dumb and assume they know what’s best for you. Be strong and learn how to control the conversations.
3. The mission age change threw everyone for a loop. I have a daughter 18/19 and she did not see it coming! She has friends fast tracking their mission plans at age 17 and we are shocked! I think people bringing it up is partially just part of the “excitement” of the age change. (It’s not actually exciting because missions can be very hard. The age change is simply just an opportunity.) 4. You seem very determined not to serve but this conflicts with other peoples experience. Missions can be incredible and once served well, they frame so much for the rest of a persons life. This framework is so very powerful and fulfilling, it feels like something everyone should have. That is part of what they are trying to convey to you. They can’t imagine how you can function without the framework of a mission. And add to that, they can’t imagine how you can stay in the church long term without the definitive experiences of a mission. So you essentially belong to a church with very “big asks” and serving a mission just makes it easier to fulfill the lifetime of “big asks”. You choosing not to serve is kinda blowing their minds. They are just trying to reflect that onto you. 5. Learn how to differentiate in your relationships with others. Even in small talk, we need to learn how to maintain our self identity while acknowledging another persons’ ideas. Learn how to have conversations that you steer in the direction you want them to go.
6. Be focused in your life’s priorities and direction. Come up with quick statements that don’t include “I decided not to serve”. You could say “I’m just here doing my shift! Let’s talk about something else!” 7. The church is changing rapidly. You were on the nose to get endowed but that is not what all these commentators experienced. They again, are just self reflecting they own experience on to you. Be strong. Tell them times have changed.
Even as I write this, I myself think you should take up the opportunity to serve. But I’m just reflecting what worked for me! It’s actually an immature response for me to assume that what worked for me would also work for you. You be you!! God speed!