Is it ok to run usability tests with non-customers?

I'm a designer for a health tech startup that makes applications for orthopedic providers, and a recurring issue I have is extremely limited access to users for research and testing. Due in part to incredibly busy work schedules, protective account managers, and interview fatigue, opportunities for valuable facetime with users are sparse. This is especially relevant now because I'm focussed on optimized on existing service within the platform that's geared toward a kind of esoteric (albeit critical) provider role, with only about 40 existing users. But given the challenges stated above, I'll be lucky if I can nab 8 for interviews and testing; but even those users won't be reliable data set, since many of these providers belong to different health systems, and therefore use this existing tool for vastly different workflows.

A solution I'm considering is to reach out to providers in this role who aren't customers of my company, who don't use our application. I could see this being fine for interviews, but I'm wondering if this will be problematic for usability testing, since I'd essentially be gauging the effectiveness of my designs based on the performance of people who aren't the end-users.

Has anyone here experienced a similar problem? Whether you have or not, I'd love to hear your thoughts. Thanks!