re MINIX 3. It's not made for high-security. It's about high-reliability. Safety is usually a precursor to security but security can take a lot more. The core has to be designed for it like Genode is doing.

re Redox. It's a nice project I've praised before. It's an alpha work-in-progress, changing rapidly, and unclear how much of its design is truly for security. A trimmed OpenBSD is probably a safer bet than it so far given the staggering amount of review that went into it. Rust's features and a microkernel only prevent so many kinds of problems.