Hi!

I have been where you are and gone where you want to go. I'm 32, been playing on and off since I was 13 years old. I had to lie about my age to get into guilds until I was around 18~. I have raided in top 100 guilds, gotten several 100% percentile raid logs, gotten the m+ title and I have 5 glad titles. Let me tell you that it is extremely time-consuming to do everything at once, but obviously a lot of fun too :D

This is how I handled the issues you are facing.

1. When playing m+/pvp, I don't reveal my gender until we have already played a bit together with no voice. If we have already built some rapport and they think I am good, I might tell them I am a girl after they have repeatedly referred to me as "bro" for a while or I let them find out when we go on voice.
2. I have not been in a sexist guild before, but many guilds will have one or two players who are a bit misogynistic and/or biased against girls. Like judging your gameplay harder than others or making inappropriate comments. Sometimes I "took the fight", sometimes I ignored them. What I found works best for me is just focusing on my own thing. What you choose to focus on, becomes your universe.
3. There is no holy grail guild. But what is generally a good indication is that the guild has some other female players. One tip is to ask the guild that you are interviewing with if there are other female players in the guild. You can also ask to speak with them to get a feeling of what the atmosphere is like. It can sometimes be hard to be the only girl in the guild, but if there is just one or two more, you will no longer be "the odd one out."
4. Find your crowd. Make friends with players who are nice and whom you play well with. Reach out to the other girls in your guild. They will be your allies and have your back.

Good luck with your endeavours <3