How to supply 12 volt DC to all the windows in a new house build
You just answered your own question, “how do I splice 30 connections” / “breaker box for DC”.  However only one breaker box/service panel/load center on the market is capable of supporting DC, and that is Square D’s “QO” type (the one with the 3/4” breakers; NOT the cheapie Homeline!)
The trick is voltage drop.  The much lower voltage is also much more sensitive to resistance loss through the wires. That forces you into larger wires.  The good news is, economics of scale make #12 or #14 wire intended for AC mains barely more expensive than the #18 or #22 one normally associates with low voltage DC.  There’s no substitute for figuring out your current draws and doing the load calculations based on your distance. This must be done.
Note that 24-volt systems are 4 times better for voltage drop. Crunch the numbers and you’ll see.  If it’s feasible to go 24V, consider it.
A shame to only use it for blinds
12V or 24V DC systems can be an incredibly verstaile addition to a home, and can even allow it to weather long power outages with high functionality, much moreso if the house is smartly designed around that idea.
For instance, a variety of LED lighting is made for 12/24V, including the popular LED strips, which dim very effectively, and even permit RGB color.  Motion sensors are cheaper in 12/24V.
If you make this auxiliary lighting, you could have it during power outages by adding a battery to the system.  Many Internet modems/routers work on 12V, as do many TVs, and a typical Roku box or other smart-box attachment will also like 12V power.  So there you are watching Netflix in the power outage.
12/24V house-sized refrigerators aren’t worth buying, but you can run an inverter for the times the fridge needs to run.
The batteries need to be sized for the run-time you want to have, but them with some solar panels to top them up can be a lot cheaper than an installed generator system with changeover switch, yadayada.  Heck a modest battery system is cheaper than those hokey 10-circuit switches alone.
Or very small generator can be used simply to top up the batteries during period of low sun (or for that matter, your car’s alternator).
It could build out into quite a system if you wanted to.