What is a good moment to start writing?
Pithy replies aside, I think it's likely the answer you could do with hearing. Echoing what some other people have said, I recommend finding what feels like a super low stakes place to write (personal blog, a shortform feed, private google doc, emails to a friend) and work on writing a lot. I find that having a thought-out channel makes it easier to have thoughts in the first place. If you are frequently in the practice of trying to crystallize ideas, you'll get better at it.
I find that whenever I try to "do a post justice" it kills my ability and motivation. I think what's happening in my head is that I assume a mantle of "well I'm supposed to be a careful thinking, well written, master rationalist, so anything that isn't that can't get out, or else how can I act authoritative?". It helps to remind myself that my intent is not to be an authority, but to express my ideas.
Perpetual draft style might also help with getting over "but what if I'm wrong?".
Also, specifically speaking to:
I know much more than a year ago. If I explain my ideas now, I'm going to be embarrassed by it next year.
I have a lot of notebooks that I've kept over the last 5 years that contain a lot of whatever interesting-to-me thing I was thinking at the time. It's been incredibly useful to have this record! A lot of the breakthroughs, epiphanies, and slow-shifts-in-thought aren't completely unique and different from thoughts I've had in the past. It's been super useful to ask "What has changed since the last time I thought about this?" and actually be able to find a record of how I used to think, and see the difference between now and then.
If you can switch from "I'm embarrassed at what I used to think" to "See how much I've grown!" you might find it much easier to write.