All I can say from experience, is that, knowing full well that I am indeed a Kafir by definition, I also have plenty of Muslim friends, ranging from pretty casual to quite strict, and none of them have ever been hesitant to greet me warmly and familiarly (a few newly come to the US may be shy, but then, they’re also a little concerned with using appropriate language as newcomers…they relax after a little time and practice), and I consider it good form to
always return a warm greeting at least as happily and more, to anyone, regardless of faith or lack thereof myself, and to initiate one to the very best of my ability! And that includes salaam alaykum/alaykum salaam regardless of who initiated it.
I can occasionally work in some of the more elaborate, formal greeting in several languages, but hey, among friends, it’s okay, even as when a close friend spots you and says ‘Hey there ol’ pal, so great to see you!’ Arabic is a remarkably poetic, expressive language, and it really is nice to know a few extra ways to express goodwill, but it’s okay, native speakers are very appreciative just with a basic, glad welcome and greeting.
A similar situation would be, perhaps, among French speakers. Yes, they do get a little irked when visitors don’t show a little respect, but if you can even say ‘s’il vous plait’, ‘bonjour’ and ‘merci’, they are amazingly warm and open. People are, by and large, really really nice in my experience (and that’s coming from someone with a big streak of cynicism), but of course, they wish to be treated considerately! And that is wonderful.
