Oh
My mother has no fing boundaries when it comes to privacy either. And I'm, ahem, a few years older than you :)
I had to be very firm, very tactless and very consistent over not walking in when I was on the loo/ in the shower/ undressed/ dressing etc. She did the hurt look and the 'oh why are you being so mean' stuff which I realised I then 'let her off' the normal respect people show each other, and I'd end up back at square one. So I decided to harden my heart to the crushed looks and wide eyed surprise, and treat my mother like a toddler. She barges in, I say the same thing each time, and then stop and wait until she gave in and went out again. If she was persistent I would do a definite toddler rules thing... Repeat my first request for privacy like a stuck record. Nicely, but firmly. And refusing to let her dismiss it or barrel over my absolute right to privacy, personal self agency and respect for my boundaries.
Basically, you don't have to sacrifice your own boundaries as rent for living at home. As long as you do it nicely but firmly, it's perfectly reasonable to get some privacy back.
She won't like it though!
As for the other stuff, I'd perhaps get a bag you can lock, a makeup case / vanity case style thing so it's not an obvious lock, but it can be locked.
What would she say if she was going through your things and found something she was locked out of? Would she ask you about it? And if so could you have one or two lines to close the conversation without ending up having to persuade her of your right to privacy each time...
The other stuff sounds harder. I'd try and bite my tongue on some of it, as I suspect you'll find it easier to have an empathetic relationship when you're not feeling encroached on! Living together is tough when you're an adult and family dynamics are changing.
Good luck