I left my first sysadmin job after about 3 years because the advice I recieved is that if you work solo for too long, you develop your own ways of doing things, become unable to integrate into more mainstream best practices, and become effectively "unhirable". I wonder if that has something to do with your lack of success in the job hunt.
It sounds like maybe there's a lot of 'institutional knowledge' about how that 42U rack and the rest of the network is assembled that only you know. It would be expensive for your employer to replace you, but with an external consultancy that replaces a large fraction of your infrastructure with something they can manage, they could survive if you left.
If you're willing to take the risk and switch careers, it might very well be worth having that sit-down with your employer where you lay out exactly what will happen to their infrastructure when you leave.
Edit: I would reconsider how you wrote your CV.
- It's not a matter of the degree to which you fit some specific title (didn't go to enough board meetings to be the Director of IT, or whatever). It's not percent score match, it's pass/fail. Were you the director of IT, or weren't you?
- Eight years is a lot of problems to solve, and a lot of weird niche technologies to learn just enough about to be dangerous with. Those are specific examples of your ability to be really good at the stuff you've been doing. Mention it. Plus, who knows if someone else is looking for someone with experience with a weird niche technology?
- Given how long you've been doing this job, PHP dev might be too big a leap to make at one go. Might be worth looking for something halfway there, that's still perceived as similar to what you've proven you're good at.