Discarding old laptops
There's no shortage of underserved students who would put them to good use. The hard drives can be reformatted with the operating system or something like CCleaner.
What do you have on your laptops that you need to drill holes into the hard drives 😉 ? A quick format would wipe the data clean. Do it a second time and you need a professional software to even retrieve something that is coherent.
Let's be honest, what would someone want with old CAD files of a lot boundary? The only one that would want them are your clients and they already have both hardcopy and maybe pdf files.
You are putting too much importance into data that is really important only to the one who commissioned it in the first place.
The only thing that I think you don't want other people to find out are porn files.
My experience with recovering deleted files, is that success is inversely proportionate to the value of said files.