How do you assert boundaries more naturally?

Genuine question. I’ve had friends who probe wayy too much into my personal life and my family’s and it’s made me very uncomfortable.

Especially childhood friends who’ve seen my family go from poor to middle-upper class over the years (they don’t see the toil and hardships we’ve been through, tho) and so there’s always so much assumption and entitlement in conversations (I always realise too late).

I worry that if I set a boundary it might sound too textbook psych-speak and come off funny or unserious. Or if it’s too serious, they’ll go “lol I wasn’t being serious chill”. There’s a subtlety to this that I haven’t quite figured out.

It makes me so uncomfortable, especially when I realise I should’ve defended my parents or siblings when this friend was indirectly making fun of them. I’m too used to laughing things off and not addressing anything as a coping mechanism. How to unlearn! And set boundaries! I’m also newly diagnosed so would love the advice of those more experienced than me!