I'm not sure what you're trying to build, but by the end of the second course, you should be able to create a customer service chatbot that is equivalent to what others have built. If you're interested in building/fine-tuning an LLM, that's totally beyond my knowledge.
MIT posts their AI/ML Degree requirements online, as well as the courses, for free. Shouldn't take you more than a year to finish it and start reading research papers.
This learning path is pretty thin. Here's what I used to go from a Physics undergrad to doing research in industry (not in a flashy company, but doing original research and modeling nonetheless):
This learning path?
Andrew Ng's Course? (https://www.coursera.org/learn/ai-for-everyone)
Karpathy's Course? (https://karpathy.ai/zero-to-hero.html)
Something else entirely?