I agree with you that every proprietary software must be presumed to be a trojan with a backdoor but still some critical, low level free software being decades old C codebases with oftentimes millions of LoC has proven to be a double-edged sword where on one hand most of it is super optimized (just compare launch times of lightdm or GDM to e.g. regreet) but on the other hand by pure probabilistics it's more likely to contain some vulnerabilities accumulated over the years of imperfect code reviews.
Sure I believe it's worth hoping and supporting initiatives that might one day bring us to something like RISC-V smartphone with high level of hardware security that'd run something like Alpine (a minimalist distro) but based on Redox OS. Maybe that'll come true in a couple of years.
But right now GrapheneOS even despite proprietary hardware is the best option security-wise, unless you're willing to tinker with hacking together some RISC-V SBC-based device (which might even have better battery life than most smartphones by up to 60%!), but the optimization of basically any software is going to suck so badly. And forget compiling any Rust code on the currently available RISC-V CPUs. want memory safety? pick something with a VM/GC instead.