Rear license plate bolt holes rusted out. How to fix?

I bought my 2014 Prius C a couple years ago and had issues with one of the bolts being stuck back then. Today I noticed my rear plate was dangling off one of the bolts. It had cracked and wouldn't stay on the other bolt (I think this bolt was overtightened).

I've managed to get one bolt free after WD40 and vise grip. The bolt looks in okay condition (aside from the Philips heads being messed up now), but the actual hole in the car seems to be rusted. I'd like to make a durable repair to this once i get the other bolt out. I've heard of using helicoils for a true thread repair. I'm wondering if anyone has done this and knows the sizes of the drill bit and helicoil I'd need (and other tools, not sure if I need a "tap"..?).

I'm also planning to sand off the bubbling rust around the holes. I have spray primer and a paint pen that I'll use. Don't think I'm capable of proper blending and it won't be visible anyway. It's even rusted on the inside.

Thanks for any advice you can offer!