Rust - Sill and rear wheel well

[jansen.fan]

Hi Everyone,

I have a 2008 Yaris in Canada. I've had trouble over the years with rust on the rear wheel well (right) and beneath the rear door sill (right). I've always cleaned the area, gotten as much rust of as possible and then applied sealers, paint etc.

My problem is that the issue has gotten worse. Today I removed all metal that was soft and corroded. See the pictures attached.

My questions:
- The wheel well damage is purely aesthetic, correct? Might I just patch this with some fiberglass and filler? I don't care that much about the looks.
- Is this part of the sill a structural component? If so, will I need to get this welded, or can I get away with simply putting in some fiberglass?

I don't have any facilities for steel cutting or welding, so my choice is to do it myself with fiberglass or send it out. The former is my preference, but I want to make sure I don't have a fatal failure one of these days.

Any input is much appreciated!