I mean think about it. If the author self-inserted but wrote themselves as a reasonable, realistic person, would you even know to spot it as a self-insert? If the author says "I'm going to write this character as me, and I'm going to imagine myself in a vivid fantasy world and write the character as I would react", and they do an honest job of it, making their self-insert reasonable, would that be upsetting?
I don't do it myself, but I can definitely see parts of myself (and parts of other people I know) in some of the characters I write. It seems to me that what bothers people is wish-fulfillment, not necessarily self-inserts.
Now for your question, what you probably meant, and what most people think of when they hear "author insert".
Many years ago, maybe a decade, I read a One Piece fanfic that was titled something to the degree of "Adventure? Y/N". The story was essentially an isekai, starring a beautiful but socially outcast Asian high school girl with piercing eyes who was exceptionally gifted with a katana, if memory serves. She's taking out the trash when she sees a One Piece manga volume that says "Adventure? Y/N". She says "yes" and is sucked into the world of One Piece. No explanation, no buildup, no nothing. Just whoosh, into the manga she went. She immediately realizes she has no money and walks into a bar, immediately getting a job as a server before the boss has her go up and sing. With no prior prep, she sings "We're In Heaven" by Brian Adams with a voice that can move angels to tears, or at least very much entertained the crowd. In the crowd, of course, is the Straw Hat Pirates (the main characters of One Piece), who immediately take a liking to her and she joins their crew. I can't for the life of me remember what happens next, and I'm not even sure if I read it.
Now I'm not going to bash the author. It's probably the fanfic of a then-13 year old girl for whom it took a lot of courage to put herself out there. I hope this person has continued writing, and has been learning and honing their skills and perfecting their craft. That said, wow. You had pretty much every negative Mary Sue stereotype a self-insert fanfic can basically have. It was bad.
Oh, and I just remembered that when I was a teenager, I tried to write a semi-isekai novel starring pretty much myself. But as an awesome fighter with a hot girlfriend in a Dragon Ball/Yu Yu Hakusho/Lord of the Rings dark fantasy that would have made The Eye of Argon look like A Song of Ice and Fire. Thank God whatever hard drive that was on has now long been burned to a crisp.
Seth is a projection of ambition through doing the right thing... the 'right' thing is often dependent on other people's opinions on what is right. He's really my thoughts and experiences on seeking approval or advancement through external validation.
Astral is about doing the right thing because it's the right thing to do. No lectures, no virtue signaling, just do the thing. She's isolated socially and mean because she can't consolidate doing what's right vs doing what makes people feel safe.
All three characters are drawn from my experiences, observations, and thoughts. It has allowed me to work out concepts in a more nuanced way.
Man had stories dedicated to himself, ate spiders, creepily stalks a high schooler, and went to Gucci HQ.
The author even says it so himself. that he's basically Rohan.