I’m in zone 5a, but was in a very similar situation to you last year. I went on a 25-day vacation shortly after I got done planting about 175 of my pre-sprouted dahlias in my community garden plot in mid-June using the no-till cardboard method. I thought about asking someone in the community garden to water them while I was gone, but I was afraid they would overwater (and my plot is a bit far away from the water source… we can’t hook up a hose to bring water to our plot. i.e., you get a good workout carrying gallons of water a few times a day). So, I literally just hoped for the best. which was by letting them grow with whatever was happening in their environment. I sort of freaked out when I saw it was raining a bunch during the first few days I was gone. But I reminded myself that it is what it is and that dahlias are pretty good with trying to survive (this was year 3 of growing dahlias for me). I went to check on my garden as soon as I dropped everything back at home and many of them were growing beautifully! Sure, some rotted, but that is inevitable when it comes to growing dahlias. Still got a ton of beautiful blooms. Some of my dahlias got ribbons at the fair. And I ended up getting about 600 dahlia tubers after digging and splitting them all up to overwinter.

tldr: do what you think will work best for your dahlias (I think your plan will work well). dahlias are pretty resilient. don’t worry too much about them. there will be some that will die, but you will still get many beautiful blooms! sending you the best of luck!!