Yeah I finally just leveled Donny to 10 doing that stuff, and grinded out a few marriages (Chrom/Miribel, My character/Lissa, Lonq/Panne). I'm just at Chapter 10, and was breezing through the main story even before doing some grinding (I also promoted Chrom, my character, and Lonqu, levelled then up to 4 or so and reclassed another dude). My main squad is beastly. Still get dominated by some of the bonus teams though, the ones with promoted units and 50+ hp.
Still, I'm going to focus on the story for a bit now so I'm not too OP. Or maybe I'll just level up some of the characters I've been neglecting so I'm not just using the same 12 or so units in rotation.
It seems there's no real limit to grinding - is there any kind of wall or could I just hang out at Chapter 10 and max everyone out if I were so inclined? Especially with some of the level 20 teams available to fight?