Drywall (note: not stucco, not plaster, not the entirety of "finish carpentry"). There's only so many ways to do it, and there are only a handful of considerations to be made. 99% of the work is "Use the correct tape, use the correct screw, use the correct mud, sand until smooth".<p><i>edit</i> I'll add, lots of trade-adjacent work. Interior/exterior painting, certain types of landscaping/lawn care, powerwashing etc. If you're willing to work with your hands you can make decent/good money on relatively easy tasks.
Jobs that don't require a long education are usually referred to as vocations rather than professional careers. It would not make someone an expert in a subject area.<p>Airline maintenance is a good example. Quick to learn and high paying, but it would not make you an expert in either aviation nor engineering.<p>In the US, to testify as an expert in court, you need to have spent 20 years at your profession.
It would be good to learn ReactJS, Vue.js, Angular, Express.js, and Next.js for the next 6 months. You may have to start at the bottom, but you did say "lucrative career" which may give you some demand.
Not six month, but my niece did a bunch of free bootcamps for web dev starting from zero for a year, and managed to land an entry level job at around $80k salary, in a recession.
Scuba diving! Clearly, "expert" is relative but you can without much difficulty become a certified "professional" diver in just six months. Diving gear and diving courses can cost a lot of money though.
- Music production (Logic Pro)<p>- Web Developer(React/HTML/JS/CSS)<p>- Graphics Design<p>- Real Estate Arbitrage or Brokerage<p>- Language Tutor (if Bilingual)<p>- Amazon Affiliate Marketing