I would try a rebuild and then assess how things are from there. not much point tying a ribbon round a turd but a rebuild will at least be a fresh one.
Yes reinstall the os. Windows is cheap these days or free if you know where to get it or move away from Microsoft altogether. like you said above not much point shelling out on ssd when you only paid 100 quid.
or what processes are using all the resources on your machine. bandwidth etc etc etc
I would try a rebuild and then assess how things are from there. not much point tying a ribbon round a turd but a rebuild will at least be a fresh one.