Matthias should just allow Iris to return to her original dragon form. She could do a whole lot more that way. But I suppose he wants to keep her underperforming to keep her better in check and without any fancy ideas. I'm also 100% sure he could have her magic vein issue sorted out any time he wanted, but he's purposefully telling Iris it's going to take 20 years to train Ruri to do it. It means Iris will serve him willingly and patiently those 20 years, with no need threaten her continuously, nor be worried about Iris disappearing while he's busy elsewhere. It's hard to say how much of this is warranted, how much of it is simply because in his previous life he used to slay all non-humans he encountered. However, with Iris's personality being as it is, I can't help but believe that even if Matthias healed her immediately, she wouldn't suddenly turn against him or break any promises. Isn't she quite a good girl for a dragon? Which is why I called her a winged dog previously.