Honestly I think Grandpa shouldn't be cloistering himself in a training sequence leading up to the wedding. Spend your engagement months happily! Yeesh. It's not like there's gonna be wedding photos. Paintings maybe but just get someone to fix 'em afterwards XD
Also... Ten years? I mean that seems about right, actually, if we want to stop the whole cradle-robbing thing. Though I keep forgetting about that because she's pretty much 16 on paper only (doesn't act it, doesn't look it, isn't treated as it, and also isekai age-shenanigans).