I speak fluent Thai. "Sour teacup". "Spider my cup".

I don't know any farang that can speak Thai personally (not via the internet). But they exist because I know several of them via the internet.
I think only a very small percentage of farang can speak, read, write and understand Thai on the level you described.

5000+ words is really a lot .... you can have a pretty long conversation about most subjects with much less words.

The Japanese are doing much better than farang.

the best speakers were practically Thai people. IE when I was in uni in BKK my Ukrainian friend was very good. Very subtle, but was born in Thailand and at that point never been outside the country (was 22 or 23 when we met). Funny he took Thai LVL I that our uni offered (crap class) and he didn't even know any Thai writing. Although he was quiet and underspoke in general in whatever language he spoke, his handle on Thai really came to light when you could catch a glimpse of what he understood, which was everything. I find most one on one conversations are usually tied into topics and levels that are relevant to the speaker, but this guy could catch pretty much everything of teenage girls full out gossiping in street slang, or watch movies and TV and keep up. Most really good people I met had back grounds like this, or had implanted themselves outside of BKK for a while ie host family type stuff. There are many many skilled young people out there.