I'm not sure this situation is comparable because the US is already fighting a proxy war including economic sanctions against Russia's invasion of Ukraine, with US materiel and funds directly responsible for the deaths of thousands of Russian soldiers.

Pentagon estimates 300,000 Russian casualties so far. https://www.nytimes.com/2024/02/15/world/europe/russia-invas...

In concrete terms what would you suggest the US do now in order to respond to Navalny's death?

I predict crossing that line will cause Biden to do nothing different.