What point do you guys start watering a dahlia tuber and when to transplant?

Just what the title says, basically. My Penhill Watermelon dahlia tuber came to me with the eyes already active and now it’s growing shoots and roots. My question (or couple of questions) is at what point, do you those of you who start your dahlias in bags, start watering them? Advice on that is welcome as well, such as poking drainage holes etc etc.

Also, I can’t put my dahlia outside until after Mother’s Day in my zone, 6b (I think 🤔), and I live in a trailer so as you can see it’s in a very small space under my skylight and a grow light, so at what point would you suggest I move it to its permanent pot? It’s obviously going to be very big, however would still fit in the same room, but the light would be massively decreased and that room gets the most light. I also can’t move the grow light, as my actual houseplants, and some peppers I’m sprouting for the garden, need it. I was thinking that it would surely be okay until May unless it gets super root bound but I’m unsure. TIA!

PS the maranta doesn’t get spoken about lol it’s doing good except I’ve had the humidifier unplugged the last two nights so I’d have room to plug in the heater and it decided to wilt a (up until then) perfectly healthy new unfurled leaf 🤦🏻‍♀️