If maximum (or just more) flexibility wrt memory safety is the goal, I might suggest the author take a look at SaferCPlusPlus. In particular, it supports memory safe pointer/references[1][2] that can target objects associated with different allocators (including stack allocated objects) without, as the author desires, imposing the "sometimes confining" restrictions that Rust does. And in terms of data race safety, it allows you to hold "read-lock" and "write-lock" pointer/references simultaneously in the same thread [3], which adds a little extra functionality (and maybe convenience).
[1] shameless plug: https://github.com/duneroadrunner/SaferCPlusPlus#registered-...
[2] https://github.com/duneroadrunner/SaferCPlusPlus#norad-point...
[3] https://github.com/duneroadrunner/SaferCPlusPlus#tasyncshare...