@emb is correct. it helps to prevent huge segments of sentences being in hiragana only and having to waste brainpower parsing the n/adj/conj from the particles.
as for not writing them in kanji, many animal names (especially those from more foreign/distant habitat from Japan perspective) use kanji that are outside the jōyō list and thus most native speakers are not expected to know them. often these kanji were originally invented in ancient China specifically for those animals and assigned pronunciations to approximate preexisting foreign names from languages of the animals original habitats region or languages of the traders who brought the animals to China.
additionally, being invented kanji, they sometimes have a huge number of strokes. this is no problem for a highly educated young person reading a book at close distance. for an elderly person with deteriorating vision trying to read the name on a sign (at a zoo, for instance) from meters away, the kanji may as well be a QR code. by the same token, some medical providers also have exterior signs with kana replacing one or more characters in a kanji term. this is not standard, but it can be thought of as consideration for those with vision difficulties.
furthermore, the more complicated kanji are learned in high school (and some in college and just adult life) but compulsory schooling is only up to jr high/middle school. and many elderly folks could have grown up in the postwar era when many schools were not yet rebuilt.
personally i think you should use what feels right for you. Japanese society mainstream doesn’t think that way. for them, things are correct or not correct.
tl;dr - if you’re okay with lots of repetitive questions from every new acquaintance, feel free to try any persona in Japan. there are niche communities for everything (can be hard to find).
beware of Japanese ppl’s questions. they are not as familiar with the idea of asking in a considerate way. if they see someone with large breasts, they might even ask the person “do you like sex?” because many Japanese actually assume that having large breasts and talking is the same as bringing the topic of sex into the conversation.
back to “boku”, there’s one female celebrity that uses “boku”. her name is あの (ano) and her style ranges from gamer hoodies to frilly goth loli… apparel. if you fit a similar niche and/or only interact with gen Z or alpha or lgbtq folks in Japan, you might get away with using “boku” without facing a ton of questions/corrections.
otherwise, as a foreigner, every new person you meet over there will “correct” you to try to make you say whatever’s the textbook thing for your assigned-at-birth sex.
regarding pseudonyms usage irl, Japanese are more accepting of foreign sounding names for foreign looking faces. otherwise, again, a ton of questions, like “are you Japanese? any Japanese ancestors? half Japanese? quarter?” and every new person the same questions repeating over and over.
this happens so much with every part of life for foreigners in Japan, some tried to decrease meeting new people or reduce trips leaving their home neighborhood, etc, just to reduce the repetitive experience.
sorry for doom and gloom. just want to make sure people have as positive an experience as possible in Japan. that means setting realistic expectations.