Little known fact, Tolkien was genuinely a half-elf who had lived through the fall of Mordor...

I don't think that "Write what you know" means that you cannot use your imagination. Otherwise fantasy and sci fi couldn't exist at all, because no one has experienced them.

I have always viewed it more as a general concept to give you a starting point. It's not a rule. It's not a restriction. It's an encouragement to include things that you know into what you write, because they are things that you understand more deeply.

Also, if it's your first book then it's totally okay if it's not any good either! It's a learning experience. Most first books aren't! Just dive in and tell the story that is grabbing you and see where it leads. I think it's better if you don't overthink it.

I've known a few people who have tried to write and they just self-edited their entire story out of existence before they hardly started. Write first, edit later.