I think skill DCs are generally a bit too high, but that's because I like PCs to be competent. I think a baseline should be about a 2/3 chance of success, which means 8+ on the d20. Given how few skill increases non-rogues (and non-investigators) get, you shouldn't really assume that people can increase their skills. So here's what I'd base skill DCs on:
Level-based: Assume a stat of 12, trained proficiency, and no items. So your bonus will be level+3. That means that an 8+ will succeed on DC 11+level.
"But that means people with high proficiencies and high stats and bonus items will automatically succeed!" — Yes. That's a feature, not a bug. If you're a 15th level legendary crafter trying to repair a 15th level item, the question should be whether you succeed or crit, not whether you fail or succeed. Repairing a 15th level item is an interesting challenge for a 15th level character who's just Trained in Crafting.
Static DCs: Figure out at what point someone could get a particular proficiency level and calculate from there. You can also assume a bit more specialization than general level-based skill DCs, because the demands of an Expert or a Master DC should be higher than a level 3 or level 7 DC.
Trained: That's just a level 1 level-based check, so DC 12.
Expert: Minimum level 3 (barring shenanigans), and I'd assume a 14 in the stat. So DC 8 (roll) + 7 (proficiency) + 2 (stat) = 17.
Master: Minimum level 7, assume stat 16, and a +1 item bonus. So DC 8 + 13 + 3 + 1 = 25.
Legendary: Minimum level 15, assume stat 18, and a +2 item bonus. So DC 8 + 23 + 4 + 2 = 37.
Also, be very restrictive with gating actions behind higher proficiency levels than Trained. You should not need to be a Master thief to be able to disarm a high-level trap.