2010 Toyota Yaris, Rust problems - can they be diy fixed?

[Trob]

As the title says, my 2010 Yaris has some bad underside rust. According to my mechanic, there is, general corrosion to the underside of the vehicle,inner sills + floor pans. I washed it today and took a bunch of pictures to try see where the rust is. I've been googling, and the wheel wells/inner sills are particularly prone to rusting on Yaris', and mine aren't great.. The rear passenger wheel well has a hole in it now, which makes me think the rust is on the inside of the sills. I don't know how I'd get to that, maybe just spray some rust converter down it to hope that'll fix it?

For the rest, I was going to try brushing off the flaky bits and put rust neutraliser or converter on it, and then paint it to give it some extra protection.

The car is also 16 years old now.. but has 36k miles, so I feel that it's mechanically gonna last a bit longer. Just the rust may make it pointless to keep. What do you guys think is the best option? Fix it or leave it until its undriveable?