Two Ravens

The game still is a prick as far as green units are concerned.
We are overrun by dozens of blues (Matilda, Delthea, Berkut, Clive). They keep reinforcing Reinhart. So I get your green / axe misery.
I'm currently running a Nino, and if I really need a hard blue counter, Narcian.

If you have the resources to build the second one and you really want to use him, you can indeed upgrade the second one.
I think your second one's IVs are not ideal either, he could really use more speed to run a quad build.

His other problem is the lack of quality native set. Threaten def is not bad but not always useful (his defenses are not that good, he won't survive much when surrounded) and defiant speed is just too risky / situational. You'd have to invest very much in a very low tier character.

You say you don't have another better unit, have you thought of Anna ? She's a real glass cannon but with pretty decent attack once buffed / furied / life and deathed, with a stellar speed. She won't survive much (not really helpful during the tempest trials ...) but for a single fight she is good enough. And Noatun makes her really funny to play.

Another interesting option to consider would be to use a Cherche / Frederick and feed them with your bad IVs Raven's Brave Axe +.

They are really strong characters with increased movement and stellar strengh. Quite common too. And when you want to build a cavalry / flying team they will be a strong addition to it.

When I had a Five Star Raven I ended up inheriting his Brave Axe over to Anna for a quad-build. I later regretted that decision as Anna seems better with her legendary weapon.

But yes.. I think promoting the Four Star and then going for the +1 merge is the way to go. -DEF is unfortunate but it's much better than -ATK. He should hit really hard on intiation.