/cgl/ - Cosplay & EGL

So, I've decided I want to bleach my hair again. I've done it several times before, but I've only used like box bleach, and this time I want to get some better stuff. I've done a lot of research and I think I'm gonna go with Wella powder lightener and probably a developer by Wella too.

My current hair color is a medium brown, one of the middle ones in this picture (I'm too lazy to figure out which one exactly but they're all kinda similar), and I want to go light enough for the Wella t18 toner to work. My first question is, is it better to use a lighter developer (like 5-10) and bleach several times, or is it better to use a stronger one (20-30) and just bleach 2-3 times? (I don't think I'll get away with bleaching just once)
I feel like using a weaker developer and waiting a week between each bleach session might leave the hair in a better condition, but I'm not sure.

Second question, I'm probably gonna have it blonde for a while, but eventually I want to dye it pink or lilac or something pastel. If I have toned hair (with the t18 toner it should be very light), will it interfere with other colors I put in? Or do you actually need to have it toned in order to get those pastel colors?