Until your house burns down
Modern Windows (and Linux) is very hard to kill. You can unplug it all day without issue. Registry corruption and similar issues have not been an issue in decades.
Get a SSD. It will run so much faster and everything will be instant.
Swap the drive and do a fresh install. It will run like new.
Wipe your device and do a fresh install from scratch via USB. It will run like new.
What ram do you have?
I do that as well
You act as though you somehow have more knowledge than everyone else. They problem is that you don't understand encryption and permissions. You can't just magically make something unreadable by programs with the same permission level. If you encrypt it there will need to be a key to decrypt it. That can could conceivably be encrypted with a password but that would require someone to enter a password. If they don't enter a password they key will be stored plain text so anyone could easily decrypt your messages. Programs running as a user have the same permissions as that user. Does that make sense? You can't just make something selectively unreadable with the current security model. I guess you could try to implement some sort or privileged daemon but that would open up more issues than it solved.
I would have a problem if Signal claimed that the desktop messages were encrypted at rest. However, they don't make any such claim. If you are concerned about security I would recommend running everything in virtual machines and flatpaks. This way the chances of something misbehaving in a way that causes harm is minimized.
It is cheaper because you are the product. Maybe find something used.
And also Chinese
You shouldn't use Android then. It is way worse
Its been around for millions of years but because it is slow it doesn't ever get press coverage
It is encrypted