Of course, the elephant in the room for all of this is that finding magic (+x) ammo is, and has always been, a disappointing prize to get. Sure it's useful and all but ultimately, let's be honest, a bit




. Happy birthday! Have some socks.
My solution - Make any arrows that are magic have a quality other than a bog standard +x - arrow of dragon slaying, arrow of undead smiting, arrow of sleep, web arrow, etc. Then they're magic regardless of the bow, and actually add some colour and value.
But then I always have Hank the Ranger's bow in my head when I think of magic bows, that makes its own magical arrows appear when shot (you would have to have it have x number of charges per long rest to balance it out, say 20, so a quiver full' worth).
Problem solved?