There are plenty of options for personal computers; you have to make the choice to go private and personal.
I built my own desktop, which remains very common and is relatively easy to do. I have Linux and Windows on it, and use Linux nearly 100% as I agree I don’t like ads etc. I use a Firefox with ad blockers and don’t get ads; I use lots of open source software even to access services like Youtube (Free tube).
There are also even linux laptops, and the Frame.Work laptop which is fully modular and bring your own OS.
There are open source OS for phones.
You’re right about the corporatisation of the internet and services, but it remains up to users to vote with their feet and chose to take back their privacy and person computing.
Linux is at 4% of desktop users in recent months - that is many millions of people actively choosing to exist in a space where they control their personal computers. People don’t need to remove computers, just chose to set them up to be what they want them to be.
Just on the KDE front, I’m assuming you’ve optimised your KDE set up for your PC?
If not, first open your Settings app and in the search box type “Effects” - disable all the fancy desktop effects.
Next, if you’re on X11, go into the “Display and Monitor” section and disable compositing (you can also temporairly disable this with Alt+Shift+F12 to see what impact it has). This option is not available in Wayland; but you may be better using X11 if you don’t have a dedicated GPU? I’m not sure I’d be messing with Wayland on an old laptop; I’ve had serious issues on a high end PC - definitely improved with 6.1, but I’m using X11 still.
But KDE 6 isn’t as svelt as KDE 5 was, so even optimised it may just not be up to the job.
XFCE is a good shout, and should run nicely on a 2013 laptop.