Hmm, I don’t remember it being hard to cast between functional types although you might be trying to do more complicated things than I did since you’re coming from JS? Since C# has a long history of big updates, I suspect some of those features are sort of antiquated and should be avoided, but it’s probably not obvious which ones are which, especially if you’re new!
As for syntax, I refreshed myself a bit and I actually think your code is pretty readable (I wasn’t familiar with the expression body syntax). The readability thing gets way better as you get used to a language anyway. Once you’ve been working in C# for a while I’d bet you’ll be able to tell what all that does at a glance.
I think another thing to keep in mind (which is something I was grappling with in Kotlin) is that when you’re using higher-level languages you’re fitting way more meaning on a single line, so it makes sense that it takes a bit to read; it’s very information dense.
I wouldn’t fret about it too much. There’s always a warming up period with a language and in time you’ll get better at working with it and knowing how to avoid its pain points.