I'm looking for smooth mouthfeel for meatballs. Does anyone have an opinion regarding using a mixer rather than hand-blending the meat?

Jann, I hope it helps. I'd love to know what works for you. E E

Hi Jann, I would agree that overworking can make the meat tough. Here are my ideas for aiding a smooth mouth feel: ask your butcher to grind the meat finer. If possible, use meat with a higher ratio of fat to meat, say 20% fat. You can try incorporating some gelatin (hydrated with some water) into your meat mixture, or you can try incorporating some ricotta into the mixture. Or use a beaten egg in your meat mixture. All of these techniques should make your meatballs more tender and hydrated.
I suspect that recipes that instruct to soak breadcrumbs in milk before shaping the meatballs are also using that step to hydrate and soften the final dish.
Browning followed by a simmer in a sauce will make them tender too.

My husband's Swedish grandmother's recipe for meatballs calls for using a mixer for blending the meat mixture with the bread soaked in milk, sautéed onions, etc. They are then baked before being simmered in a broth/cream gravy. They are delicious.

Hi Jann, this recipe produces extremely light, moist meatballs. https://food52.com/recipes/22467-rao-s-meatballs -- might be worth reading through. Hope it's helpful!