Spaghetti on the Wall...?

Hello, everyone! Looking for collective Author Wisdom from the good folks here. :)

I am a self-published author with four published books. I sell direct from my website and see modest sales, but I would like to get serious with my author career and expand my reach if possible. Because of an unforeseen job, I now have $500 extra this year that I wasn’t planning to receive, and I would like to use that money to invest back into my business.

But…I don’t want to randomly throw spaghetti at the wall and see what sticks! So…what are some courses/training programs/etc. within that budget that you would recommend to an author who’s seeking to maximize the business and promotional side of writing? I’d love to learn how to advertise effectively, attract more direct-sales customers, and just generally grow my business. Thanks in advance for your recommendations!