To put it another way, if I think of a craftsman who is solidly competent.
I imagine prof and expertise, so +4. Considering +3 prof bonuses aren’t available until 5th level (aka rare and special), that’s our ceiling.
ability wise…I would allow for a +1…which is already above the curve (aka this person isn’t just skilled they are naturally talented…which is already a bit much for our standard competent craftsman but let’s allow it).
so that’s a +5, so DC 15 with take 10. So we need to respect that to craft dc 15 goods and expect to succeed at your craft, you need to be highly trained and naturally talented.
so how about a crafting master? For that a +3 x2 for expertise makes sense, so +6. Now…I would still argue most masters would have a +1 ability…but I’ll stretch it to a +2. This means to be a master requires lots of expertise AND to be very very gifted. It’s easy for PCs to forget how rare 14s in a stat should be if we consider the standard population.
So that’s a +8, or DC 18 take 10. So a “simple” sailboat is something that only master boatbuilders would make, standard boatbuilders would do it as a special project to enhance their craft…but would assume failure the majority of the time. That does not feel right to me.
Now DC 20 for exceptional items works for me. That’s something a master would struggle with, fail at a few times, and then succeed. Basically a true masterpiece.
to make those regularly (aka +10), would require very high level skill or insane natural talent. That’s your once in a generation craftsman…or a PC
But that’s bounded accuracy for you, realistically most things should get +1 - +3 ranges of difficulty, as +5 represents a HUGE increase in difficulty.