Looks like you already have a solid foundation right there. Ryoma and Linde get you well covered in red and blue; you are just lacking green. You can try on Minerva's banner; she's pretty good right out of box so whoever just started the game can benefit a lot from her. Or you can wait for the Hero Fest with Julia, plus Ike (though you are not in desperate need of him since you already have Ryoma) and Ninian can also be quite helpful. Even the summer banner with S!Elise and S!Xander aren't half bad; you get higher chance for a green 5*, and S!Elise is a great green mage.
As for feather investment, maybe your priority would be to promote the starter trio to 4* to get your arena going and brings in feathers/orbs consistently. Besides that, get your core team (Ryoma + Linde, and hopefully a good green) ready (lv40 and some suitable SI) before Xander and Camus GHB reruns hit; you do not want to miss those. Fortunately, judging by the schedule we know for now, you should have plenty of time to prepare for that.