Reading theough the spark tutorials and examples seems fairly similar to the rustlings excercise. I find the ways spark prevents problems, like forbidding side effects, is more onerous than the borrow checker making me specify a lifetime every so often. I'm not saying you are wrong to prefer spark, just that people with different backgrounds will find the learning curves different. You have been programming longer than I've been alive, so you certainly have a different perspective. I started with c64 basic then c on the amiga 500, then x86 assembler on 386, them turbo pascal, then turbo c++ on the 386.
I can't argue that spark isn't more mature for formal verification, but it feels like formal verification relies heavily on correct contract specifications with no way to measure when your contracts are correct. Unit tests and coverage at least give me a little feel good that I wrote enough tests for my rust.