Is it true that estonia is hima's self insert?
I was very curious to research this, because I've always heard about it (like even back when I first got into Hetalia in 2011) but never actually saw actual proof.
Kitawiki doesn't really give a solid proof to backup this statement except for pointing out how the way Hima drew/draws himself shows resemblance to Estonia. And that is definitely true, albeit a mere observation:
The only other reference Kitawiki mentions to backup this statement is a blog entry Himaruya posted in 2009 where he jokingly says that Estonia is the coolest creation in the manga:
[You can find the translated version of the post here, the chart is massive!]
Being familiar with his humor, yes I can see him saying that as a reference to the fact that Estonia was at least partially inspired by Hima himself.
Another observation I've come across through the years that I agree with, brings up another point which is: Estonia is put through a lot of stuff. For a relatively niche, secondary character, Estonia got a few funny stories focusing on him that don't really have anything to do with him being the representation of Estonia: he's the owner of the mochis and he met the Nyotalia girls in a dream.
If you think about the nature of Hetalia, this is a level of randomness that is pretty much unrelated with either history, culture trivia or culture clash. It surely is suspicious that he decided to put Estonia, (the character Hima apparently shares resemblance with) through it all and not anyone else.
So yeah, while the rumor doesn't really have 100% solid evidence (like, I don't think Hima ever said it word by word, unless I'm missing some piece of information, like I admittedly haven't checked Hetalia Collezione), there is definitely room to believe it is true.
Japan also is sometimes used as Himaruya's sort of self-insert in the arcs where Japan finds himself in a culture-clash situation with another Nation.