I want to chime in on a couple of points here. Not an expert but I own two steel sailboats and have looked at a few more when searching for what we have.
IIRC the rule of thumb is that 10 gage or 1/8" steel is about as thin as you can weld without too much heat distortion. Again generally speaking, scantlings for 10 gage work down to about 40 feet, so below that 10 gage is "overbuilt" which is why so many small steel boats are proportionately heavy.
My small boat, 33', is 10gage. The big boat, 44', is 1/4" first 3 plates then 3/16". But I have seen much bigger boats in 10 gage. Here is a link to a relatively famous 44'er in 10 gage. (3mm)
1989 Custom cruising ketch Sail Boat For Sale - www.yachtworld.com
We looked at a couple of Colvin Gazelles. On one you had to crawl through the engine room to the aft cabin.
We also looked at a big (52?') Colvin ketch, damn near a schooner. Deeply raked masts, I mean Deep. That boat was 10 gage! Talked in person to the original owner/builder and I queried him closely on that point.
Her saloon was immense with a huge curving leather setee. You could waltz in there, with no handholds. Getting to the aft cabin you squeezed past the engine room, then crouched very low, nearly on your knees. The main bunk, a double, was to one side. IIRC the foot of the bunk was behind a bulkhead sorta like a quarter berth. We couldn't figure how to get In or out.
So, yes, 10 gage boats are out there and fairly common and yes some Colvin interiors are "interesting."