I like:
Aaliyah Alexis
Aaliyah Lexi-Marie
Or:
Alexis Maliyah
Alexia Marina (Marie plus Selena and still beachy meaning)
Lexi Marina (so pretty! Prob my fave of the second list)
Alexa Marie
Alexia Serena or Alexa Serena (prefer this to Alexis Serena because I don’t love the S’s back to back)
My sense is you love Aaliyah the most. I think Alexis Selena to Aaliyah is just about OK - but if you’re going to do that, I think you might as well just use Aaliyah as the name. Alexis has so many more intuitive nicknames and the spelling & connotations of Aaliyah is so distinct (first is Greek, second Arabic/Hebrew). I think it’ll be a pain for her to have to say, “my name is Alexis but I get called (the very different) Aaliyah because my middle name is Selena” (which only just about makes sense). I’d just go for Aaliyah in the top spot if you love it the most and want it to be the name she goes by. I also agree that having the honour in the middle spot can be more special for a kid (and where they still get their “own” name).