Just my two cents:

I will never, ever trust anything made by Google. seL4, sure since it’s both formally verified and the kernel itself is a very small codebase (you still need to be careful of who’s making your userspace programs with any seL4 OS though), but there are way too many places to hide backdoors even in open source projects for CIA/NSA backed Google to ever be a viable option for security. It’s something that the UnGoogled Chromium and UnGoogled Android projects have proven time and time again because even as projects with the express intent to remove Google dependence, they’re still finding existing Google stuff in weird places and are adamant that they’re never sure that they’ve removed everything.