What's the risk of that being done poorly by you, versus having better control over these physical switches?
What "normal OEM support" (like which company, for example?) has been good at regularly releasing security patches and keeping on top of various vulnerabilities? Especially past the 1-2 years after the release date of the device? Especially if you consider those companies that without any real consent from you install dozens of their own adware and spyware on each update that you can't even remove (like you can't remove system apps in android, only "disable" them, to make the phone nag you all the time to re-enable them).
Not having to rely on the joke of the "OEM support" from the big companies is the very reason to use these open phones.
For reference, I am typing this on a laptop from 2008. I am not worried about OEM support because all of the hardware support was upstreamed long ago.
Personally, I consider any code produced by Google to be hostile malware. Meanwhile, I'm quite competent with managing a Linux machine. For me personally, the choice is easy.
The current crop of Freedom Phones are targeted at people like myself who can break things and fix them ourselves. My hope is, in the future, you will be able to install vanilla Debian for example on your phone, which I think even the more paranoid of us would consider safe.