You know your customers better than I do, but I'd be reluctant to sell a twelve-year-old laptop with an unsupported operating system to anyone. In your position I'd slap Sequoia on it and keep it as a daily driver to stay more-or-less fluent in the Apple ecosystem, which is exactly what I did with mine and it worked out very well - I've been using it with various supported and unsupported versions of macOS for the last eight years and it's never missed a beat.
If I hadn't retired five years ago (yay!) I might still do the same but I'd probably also treat myself to a MacBook Neo as a business expense. They're cheap, gorgeous and an excellent way to keep in touch with what (some of) your customers are using.
A word of warning about Linux on this machine, since I believe we have the same model. I briefly swapped out its SSD and set it up with Ubuntu (that's what happens when you have time on your hands) and it was a bit of an uphill struggle. There were a few hoops to jump through to get the bootloader to recognise the SSD, then rather a lot more to get the right WiFi driver installed and stable, and I never did get the camera working well. It was smooth and fast when I was done but it always felt a bit delicate, in a way that macOS never did. It was a relief when the SSD popped its clogs and I had an excuse to go back to macOS. Ubuntu on every originally-Windows machine I've used was much more robust and far less painful.
