Since childhood, I’ve always held a pen in a slightly “wrong” way.

It's more natural to add "always" when saying "Since _____, I've [always]..." when you're talking about something that you continued to do since a certain time.

Because I attended a calligraphy class, I assumed I had been taught the correct grip.

Was it just one calligraphy class session? If so, this sentence is correct, but if it was a semester, period, or series of classes, it would be better to say "I attended calligraphy classes".

Since my childhood, I’ve always held a pen in a slightly “wrong”unorthodox way.

子どものころから works perfectly in Japanese, however since English is a low-context Language: natural Japanese usually ends up looking unnatural when directly translated into English. You will eventually reach the point where you no longer have to directly translate, but for now, imagine that you are 蓬莱純 from Witch Watch. The unnatural manner that he speaks in is what English would sound like if directly translated into Japanese, so if you can "reverse-engineer" his manner of speaking, you can use his weird Japanese as a reference to produce more natural English.

Furthermore, since there is no objective assessment on how one should hold a pen, your method is not wrong, but simply atypical—so "unorthodox" would work better here.

MIn particular, my thumb btends too far inward cave slightly inward while writing.

Edited to make it a bit smoother.

Because I attended a calligraphy class, I assumed I that had been taught the correct grip.

So when I realized that mine wasn’t exactly the textbook styledeviated from the norm/expectation, I was surprised.

Just another touch-up

Another possible alternative is: So I was surprised to discover that mine deviated from the norm/expectation.

Perhaps in Japanese this would translated as: "びっくりしたのは自分の持ち方は普通のと違っているのに気づいたときだ。"

(下手な日本語でごめん ✨)

For me, however, this way of holding a pen feels the easEven still, the methods that I have been using thus far has been the most efficiesnt and causes no strain even when I write a lotfor me, personally—and have not exerted a significant strain upon my hands even during prolonged writing sessions.

Hopefully this doesn't sound like gibberish, but it is basically attempting to convey those same thoughts, just with more natural English

I once tried switching to the “right” way, but it felt so unnatural that I couldn’t keep writing for longexperimenting with writing according to the manner in which the sources around me recommended that I do so, however it felt completely unnatural and was beginning to lead to discomfort, so I stopped.

Just another touch-up

Since everyone’s bone structure, muscles, and habits areanatomy is different, it doeisn’'t make much sense to insist on one standard wayfeasible to assume that any particular method will be optimal for everyone.

In fact, even when wringing out a cloth, I use my right and left hands the opposite way fromin a manner opposite to that of most people.

I added "even" for emphasis and continuity with the last sentence. Using this word helps to maintain the momentum between sentences, and build toward a certain "climax"

Some may judge those who don’t follow the textbook methoeviate from the standard, assuming it’s due to a lack of education or practice, but I believe the most natural way for each person is the bestat each person should utilize the methods that work best for them.

Some people may judge othoseers who don’t follow the textbook method, and assuminge it’s due to a lack of education or practice, but. However, I believe the most natural way for each person is the best.

Personally I'd say it this way, but what you have is fine.

How I Hold a Pen: Comfort Over StandardConvention　私のペンの持ち方

"Comfort over Standard" is awkward. You could say "Comfort over the Standard" instead. However, I like "Comfort over Convention" because it's a cool alliteration and makes you sound smart. 😎

Since everyone’body's bone structure, muscles, and habits are different, it doesn’t make much sense to insist on one standard way.

"Everyone's" is a contraction of "everyone is" and is wrong here.

In fact, when wringing out a cloth, I useturn my right and left hands the opposite way from most people.