Niobium is very slow to work harden. While it will stiffen up a bit, you should still be able to open your pea pods without annealing

I've used these pads and the trick isn't so much about which grit (all will work) it's more about making sure the discs are on a flat surface and the pad is held parallel to the disc. Don't use the pads edge as it will always hit the edges and in the grooves. It also helps if the embossed parts are deep. Some shallow ones are harder to keep from hitting the edges too.

Welcome!

I would use a minimum of 1/0 maybe even thicker. Make sure you use a lot of good lubricant and go slowly--that's the key with titanium.