I would definitely trasplant probably before flowering and maybe a few days so they can get over the stress before you switch them to flowering.

As far as the nutes - I was doing some research and talking to some other members and I found out that switching nute numbers can cause additional unwanted stress so I plan on using the same formula when I jump right into flowering. I'm using 14" pots and I'll probably transplant when they get up to the 2 foot range

My plants got a little rootbound in their 8" pots and it's definitely not something you want to deal with lol. I was amazed because they didn't look big enough to be rootbound, but when I pulled them out I could definitely see a MASS of roots collecting at the bottom and trailing all the way up every side of the soil basically making the pot one big rootball. They're still recovering a bit and should be good as new by Monday at the latest.

Get 5 gallons if you have the space for it, you won't regret it and your plants won't thank you enough.

the 15-30-15 is ok, the 30 is kinda high but as long as you're giving them low doses like you said, you'll be fine. Remember, they use up Nitrogen first and foremost.

How often are you watering/how much do you water? (out of curiousity)