20-30amps of DC requires very little copper if it's high-voltage. It's low voltage that requires fat copper, not DC vs. AC.

The same number of amps will require the same cross-sectional area of Copper, for both DC and AC.

You can use less copper for the same power with higher voltage as a reduced current is required for the same power. This is also independent of AC/DC.

(nitpicking) That's correct if you take into account the skin effect when computing the cross-section area (or if you design your transmission line to avoid it. Better solution). But at 50/60 Hz, it's negligible (the "skin depth" of copper at these frequencies is around 9mm)

Except for electric cars there is very little consumer facing HV DC. Solar panels are 12/24/36/48V. With a 20-30 meter cable run it makes sense to convert it to higher voltage at the source. Since it's going to be used in traditional AC grid system, skip a step and convert it to AC as well. Kills 2 birds.

True. And HV DC (more than 50 volts or so) is more likely to kill you than HV AC if you grab a live conductor. Just the same, if I were building a new PV-powered house today I would try very hard to wire it as a mostly-DC house with 12-gauge or bigger romex and switchable voltages at the breaker box in increments of 12, up to 48. I'd put in point-of-use inverters at the few places that needed them instead of having 100 point-of-use rectifiers like we have today. The overall efficiency of a DC house would be quite a bit higher than a typical AC home.

I live on a boat and from my experience you're better off using AC. We have 12VDC outlets and 5VDC USB outlets around the boat. Except for basic lighting they are practically useless. The 12VDC cigar adapters aren't designed for the load we put on them 80-90W (120W and they get extremely hot). And with motors you get into other concerns like the starting current can be 3-4x higher.

We have a single 3kVA inverter for the entire boat. From this we can run clothes washer, power tools, kitchen tools, and blow dryer from our batteries. Modern wall warts are incredibly efficient. You are losing, at most, 3-5% efficiency. Plus, it's really hard to find 12V anything thats any good. A 12VDC vacuum cleaner is a joke compared to a proper AC vacuum cleaner.

Trust me whatever efficiency you think you'll be saving is better off buying more solar or bigger battery bank. Keep the distribution simple, 120/220V. Point of use inverters are for charging laptops in the car.