Bleach bath help?

So next weekend it will have been a month since my last full bleach (with 30vol). My hair is yellow blonde and I'm trying to get it to a pale (inside banana) blonde before using pravana silver.

I also bought some matrix light master bleach which I will be trying for the first time. I've heard it works quicker than other bleach (previously I used Blondor soft lightening cream) so I was wondering if I should go for 15vol or something? I'll do a strand test anyway.

Also, my main concern is: is there some kind of timing rule for how long you need to bleach your roots for compared to the mid-lengths and ends? The last time I did my hair, my roots were slightly lighter even though I did them last, about halfway (time-wise) in. Should I also consider using a lower vol on the roots, like 10 or something?

Finally, since I won't have help doing my hair I thought the easiest method would be to dilute the developer with water instead of shampoo so it runnier, then I can easily smother it all over my hair quickly, avoiding the roots and get it straight in a plastic cap. (It takes me ages to cover my hair when I try to it really strategically). Then after a certain time has elapsed I could go back and put some bleach on my roots. Is this a good idea/method? I'd really appreciate some input.

Thanks 🙂