how do i use python for nlp and semantic seo without a big budget?

Everything you need is free. Google Colab for the environment, spaCy for entities, and sentence-transformers for embeddings. The most useful cheap project: export your queries from Search Console, embed them, cluster them, and see which clusters have no page that properly targets them. That gives you a content gap list based on your own data instead of a tool's guesses. Don't bother with paid NLP APIs until you've outgrown that.