I recently picked up an angelface floribunda bare root rose bush to grow in a large container on my porch. I am preparing the soil right now. It's a mixture of potting soil and rabbit poop. However I realized that the rabbit poop contained gnat larvae. I made a mixture containing two cups milk, 1 and 1/2 cups of hydrogen peroxide, 1 tbsp of baking POWDER (I totally just learned baking powder is baking soda + Cream of Tartar and cornstarch) two packs of an herbal tea containing mostly chamomile and 8 cups of water.
My questions are as follows;
how long should I let my soil mixture sit on my back porch before I mix it with the rest of my soil medium?
Should I test the pH of my soil before mixing it or after?
Should I mix it with more potting mix or should I use something else?
(I'm planning on putting rocks at the bottom for proper drainage)

No rocks on the bottom, it has been found to be a practice that contributes to poor plant health and harbors snails, slugs, bugs...BT is a better choice to help deter gnats and such. I just use old screening to place over the drain holes. You said a large container, how big? How deep? extra depth would help with keeping the roots from standing too long in moisture, but let me see, North Texas, zone 6 would indicate you are in the region of Dalhart, Perryton, Stratford...up where moisture isn't a problem. Hmmm, most bagged potting soil is a neutral ph, that close to the caprock caliche and limestone, you probably still have a fairly neutral ph, maybe 7.0. I would imagine you would take the ph after you mix the soils, but they won't be a stable ph for some time - especially since you used a form of baking soda that lowers ph. You will need to wash your concoction on out of what you treated it with and allow to dry somewhat.

Update.
It's POURED rain for days after I planted my bush and I'm pretty sure it got dieback.
It now has ZERO canes (I pruned them all the way down) and I'm pretty sure it's dead.
I used a saw to cut down below the first graft knot.
It looks dry in the center but the main trunk still seems to have some green despite the fact the roots are kinda red looking. And they haven't grown any more little shoots.
I've simply buried the entire plant sideways in the dirt and I'm hoping for the best.
I don't think anything will come from it but shrugs. I don't want to write it off just yet.
Also I found out I live in zone 8b

I hate that you cut all the canes off.

I grow roses from the un-rooted canes all the time... I just shove the pieces into the sand, water once and walk away.

I don't understand what all the pantry stuff has to do with growing roses...

Rather than a discussion of these weird baking ingredients, we actually need information about what kind of soil you are dealing with.

Sand? clay? gumbo? caliche?
Lot of different soil types in texas...

Maybe tell us what city you are near, and we could look this stuff up ourselves...