[Question] Grandfather (RIP) is purchasing me a watch up to 20k. where should i get started?

I'm in a unique/fortunate situation. My grandfather's estate just informed me that I can purchase a watch for my upcoming birthday. It can be up to 20k. I don't know much about watches. I have a Rolex (blue datejust), a Cartier (ballon bleu), and a Tag Heuer (black dial carrera). All of these were 3k-10k when I purchased them.

Watches in the 10-20k range have never been attainable for me and I'm feeling overwhelmed. I have to make this decision in about 2 weeks so I do have some time.

Can someone give me some tips on where to get started, where to look (is purchasing online safe nowadays?), how to know if I'm getting a good deal, etc.