I wouldn’t worry about balanced at this price point. Here’s why:
It’s still potentially forward-thinking to balance mod your DT1990 if you can do it safely, because it will open up the possibility of a wider range of amps to use down the road. Balanced amping does not inherently sound better, though.
The G103-s is probably the best sounding amp you have on that list. I haven’t heard it myself, but I trust those who say it’s the real deal at the price point. You don’t have to match it with a Topping E30, though. You can use a Zen Dac V1 or V2 with it just fine.
You will get sound.
I like the Zen DAC just fine as a DAC. I’m not nearly as high on iFi’s entry-level amps.
This is a wrinkle I didn’t expect. Do you need transportability in your setup?
If you’re gonna order Schiit gear, I would strongly suggest the Asgard 3 amp. IMO it’s the best <$200 headphone amp on the market right now. With shipping and customs, it might be close to the price of a G103s (?). My recommendation for you, if it’s isn’t too much money, would be to use a Zen DAC as a DAC and connect it to either a G103s or an Asgard 3.