We've done well with transitioning our relationships from parents-children to parents-adults. My husband took a little longer than I did, but we've all come to a good point. It's our house, and we do have some rules that must be followed ... but we've figured it out together. The problems are never the things you think they'll be; for example, one issue for us was who'd be home for dinner ... we didn't like guessing how much to prepare or whether our vegetarian child would be eating.
She's a senior in college and has more than 10% of it already, and she's reading about being a landlord /bugging her dad to teach her about simple repairs around the house in anticipation of buying a rental house.
Sounds like a good example of, Ultimately it is your house, and you are entitled to make the rules.
My husband's personality is much like yours: he has to have his time /his space, and he needs to know things ahead of time. In terms of adult children though, it hasn't been a big problem. Our kids are good about letting us know things ahead of time ... and that's the biggest thing: communication.
Eh, nope. Some 30 year olds DO need help making a plan. And 30 year olds are certainly not "almost middle-aged".
General rule of thumb, yeah, I wouldn't treat my children this way ... BUT I remember a student of mine (high school senior) from 2-3 years ago; her parents kicked her out with just her clothes. Had to leave her phone, her car, she clearly had no way to support herself. HOWEVER, I support the parents in this case. The girl moved out to live with her boyfriend, a boy who has been arrested multiple times for selling drugs, high school drop out, no job, estranged from his own family. He had already dragged her into the world of drugs. They wanted the girl to see what type of life he could provide for her. Awful situation. The parents told her she could always come home, but they would not help her create a life with this guy. How'd it end? Police raided their apartment, found loads of drugs + pot being grown in the closet. The guy at least had the decency to tell the police it was all his so she wasn't arrested along with him.