Not specifically USA answer, just some general thoughts that could probably work anywhere:

• A generous safety net, maybe UBI, so anyone can take as much time off as they need to requalify for a trade
• Very cheap (but not free) training
• Easy path to education and training, there shouldn’t be too many hurdles between “i’d like to do that” and “i’m officially an apprentice”
• Enforcement of health&safety regulations - trade jobs often have lower life expectancy for being physically hard
• Ensured healthcare and retirement
• Paid maternity leave for self-employed
• General support for unions, while pushing back on some of the excesses (like limit on the number of members)
• Edit: Simple taxation. Not necessarily low taxes, just make them simple to calculate and pay. Tradespeople don’t have an accounting department.