A couple gotchas that basically all Lisps-on-other-languages have to keep in mind are: variables, since Lisps usually don't have the same scopes as anything imperative (with the ‘let’ blocks); and function names and possibly functions-as-variables, since Lisps are liberal with both. As a rule, you'd have to implement your own variable scopes and name mangling for where the semantics don't match. Plus macros are an obvious addition—IIRC Hy still has them in separate files (though I might confuse that with Fennel). And if you wish to have keywords as a separate type, that's usually entirely on you—afaik Clojure went full-in here, others not so much.
> reading unfamiliar syntax often seems to be a sticking point for people considering/learning a new language
Precisely the opposite of my and many other people's experience. Remembering whether blocks are delimited with braces, indentation or e.g. ‘then/end’ is a no-brainer. Learning the subtle differences in semantics, the type system, the library, package system and build environments—that's the bitch.
Consider that there are dozens of even kinda-popular Lisps, despite them not having much syntax in the first place. They don't just differ in the function names.
You seem to be imagining a ‘universal-ish intermediate language’, so to say, that would permit people with different experience to quickly adapt to an environment of a different language. Not possible, because a) semantics is where the difference is, you'd need a full-blown language translator, which still can be lossy or incomplete (e.g. no types in your standard Lisp); b) one would still have to deal with all the non-language parts of the environment, like the library, packages, the build system.