However, the amount of teaching material focusing on software verification using Lean is frustratingly small?

Coq has FRAP and CPDT [1], which teach you most techniques that are used in industry. Also Software Foundations.

AFAIK, there's a version of Concrete Semantics using Lean [2]. But that's still just semantics.

Any other materials focused on software verification, not on formalizing mathematics?

[1] http://adam.chlipala.net

[2] https://news.ycombinator.com/item?id=22794533