I kept Rex as an attacker and left the tank and healer duties to other drivers. I think if you dont want to be an attacker youre better off changing your characer to be someone else as you cant take Pyra from Rex (
the same is true of other drivers and their special Blades, they seem to want specific characters to be tanks and healers) so he will always have at least one attack blade so you might as well focus on him being the attacker. Once you increase your luck crack open any cores you have, get some more options for Blades as soon as you can, later on you will also
get more drivers so your options will expand with them..
Regarding the Noppon:
He does get other Blades, he can eventually wind up with three but the 3rd one is an optional side quest, much later in the game. I didnt change Poppis element ever, I had a broad enough range with other Blades that I didnt need to but that might be different for you depending on what other Blades you managed to resonate with.
Finally, only look at this is you really dont mind spoilers, it covers some story adjacent stuff you havent gotten to yet: