>What can be done to mitigate this?

Buy hardware used by government computers that are rivals to your country.

So if American, buy Chinese CPUs and install Chinese Linux or HarmoneyOS Assuming there is nothing you are doing of interest to them, as that will also have back doors

After Snowden, one can only imagine the worst and think everything has a backdoor.

But unless you are a high level terrorist or other person of interest, no state organization is going to target you at this level

P.s. you say no one can trust closed source, but a lot of open source is maintained by one or two people or a small group, just takes infiltration by one or two trusted contributors to push malicious code in and unless someone looks and finds that code amongst millions of lines of code, may never be discovered (more so as mainstream media won't publish any thing)