Parents don't see me as an adult (I'm 21)

Yes I still live with them when I'm home from uni but saying "just move out" isn't a viable solution in today's economy
I've been home for two weeks now and it's driving me insane the amount my parents still seem to be convinced I'm about 14. Like I can't leave the house without telling them where I'm going, who I'll be with etc, even in the middle of the day. They try to open my parcels and even if I get to them first they hound me about what's in it and treat me like I'm being shifty if I won't tell them. Watch what I'm eating and give me **** about it if they think I'm eating too much/too little. Make comments on the times I go to bed or get up. Try to look over my shoulder when I'm on my phone or ask what I'm doing. It's driving me mad that they just seem to have no boundaries or give me any privacy.

You say moving out isn't viable, but it doesn't change that it's simply the right answer if you want independence and privacy.
Your parents are unlikely to change, you're unlikely to be 'promoted' to house adult and any objections to questions, diet, parcels etc will always run straight into 'our house our rules'.

By law you became a adult at 18.
You are old enough to decide what you want to do in life and if it means moving out and finding a house for yourself then do so but make sure you have can afford it as you have plenty of bills to pay. Sometimes it is cheaper to stay with your parents and save up until you can afford to hopefully buy a house 🏠 🏠 🏠..... But only you will know if you can do this right now and remember that they can't stop you as you are well and truly over 18.
They shouldn't be opening your parcels etc as that's against the law for someone else to open somebody's else's mail and parcels.... I don't know if you can perhaps get your parcels arranged for you to collect yourself.??
For eating - you can go out and have meals but of course this costs money for you......

I can’t afford to move out, doesn’t give them an excuse to open my parcels, try to control what I eat, etc

It’s not really like everyone’s lining up to give a mortgage to a 21y/o full time student, I live away from them when I’m at uni and my student housing contract is 9 months

Do they try to control what you drink???

I know you are home from university and just want to rest but is getting a part time job an option?
It serves multiple purposes - you will have less interaction with your parents, you will have your own money to eat out/shopping, your parents will start considering you as an adult. Some parents can't go by numbers so being 18 does not matter much but getting a job will.