Removing old stain - How do I know when I'm done?

Purchased a new house last month and decided the deck would be the first project...

It had an ugly red stain that was peeling in lots of places, I first tried a chemical stripper and picked one up at lowes but after using an entire gallon on a small area to minimal effect I decided to switch to sanding.

I rented an orbital sander from home-depot and spent a day going over the deck, and while it did look better it wasn't the "clean" appearance I was expected. I've since gone over a bit with a belt sander and angle grinder with paint remover attachment but how do I know when it's removed enough and I can stain?

Since I didn't use a stripper on the majority of the deck do I need to use a brightener prior? I've I go get another stripper will it work better after sanding? Since I still have the red stain in the cracks and grooves of the wood do I have to use a semi-solid/solid to cover it or would a semi-transparent also cover it? Can I paint over the railings with a deck paint and not sand them since they are a vertical service.

I don't mind the work involved to make it look good, I just don't know what to do. The angle grinder attachment seems to work best (deck panel closest to wall in picture with white painted wall) but it's battery operated and takes about 1 full charge per board...), the belt sander is faster obviously but doesn't remove a ton and leaves what looks like horizontal strips (same picture as before but the boards further from the wall).

Any advice would be appreciated.