Make the horizontal 50% bigger. Looks like the tail span is good, but you might increase the chord. This does 2 things. It expands the allowable CG range. So, you can keep the CG where it is on the wing, and have a better behaved plane. The second thing is it increases the pitch damping. That is, even when it's balanced agressively the response will be slower and less twitchy. Think about holding a piece of cardboard in your hand and waving it like a fan. THe larger the fan, the slower you can move it. That's not to say elevator response will decrease, just the twitchiness. The good news is, you could try his mod just by building a bigger elevator and sticking it on there.

Can I ask how you determined that the best CG location is 50% for this plane? I move my CG on similar planes until I get level flight upright and inverted. Usually that doesn't involve going back as far as 50%.