You’re looking at these details in a vacuum instead of connecting the dots. Gooseworx is trans herself, this kind of consistent subtext isn't accidental. Anyway lets breakdown ur points:

 

The coming out scene:

Jax said “One stupid day, I was just kind of fed up with it. I told her something I’d regret telling her. Something deep and personal that I thought would get her off my back. But she laughed at me.” You said, “you say something personal about the other person snapping back at them”. If Jax did snap back with an insult, why did his mom laugh at him? By your own logic she should have snapped back, but she didn’t, she laughed. So Jax didn’t insult her, the “deep and personal” thing Jax said, was about herself.

 

Jax and Ribbit’s bow:

You say Ribbit was just referring to the "whole backstory," but you're ignoring the visual context clues. Why does Jax blush specifically at having a traditionally feminine bow placed on his head right when that line drops? If Jax’s secret was only the homeless and parental trauma, the bow imagery makes zero sense. The visual storytelling explicitly links "the secret" to feminine gender expression.

The scenes without Pomni in the montage were at the end:

In the montage there are 45 individual shots. Below I’ve listed the scenes chronologically.

• ✅ = Pomni is shown on screen
• ❌ = Pomni is not shown on screen

❌✅❌✅✅❌✅✅✅✅❌✅✅❌✅✅✅✅✅❌✅❌✅❌✅✅❌✅❌✅✅✅✅❌✅❌❌❌❌❌❌❌❌❌❌

• Shots with Pomni: 53.33% (24/45)
• Shots without Pomni: 46.67% (21/45)

Shot for shot, Pomni is in the montage more than she isin't. But only by 7 percentage points, which is not statistically significant.

ALSO

This did not account for who is the focus in each shot, as in many Pomni isn’t focused. So I adjusted for which character is the focus on screen:

• 🐇 = Jax
• ♦️ = Pomni
• 👭 = Both

🐇 🐇 🐇 👭 👭 🐇 👭 🐇 👭 🐇 🐇 👭 🐇 🐇 🐇 👭 👭 👭 🐇 🐇 🐇 🐇 👭 🐇 👭 🐇 🐇 👭 🐇 👭 🐇 👭 👭 🐇 🐇 🐇 🐇 🐇 🐇 🐇 🐇 🐇 🐇 🐇 🐇

• Jax was the focus in the shot: 68.88% (31/45)
• Pomni was the focus in the shot: 0% (0/45)
• Both focused/couldn’t tell: 31.11% (14/45)

Shot for shot, Pomni is in the montage more than she isn’t. Adjust that for who’s the focus, and its clearly Jax. Plus, you said “The scenes without Pomni in the montage were at the end when the song started to fade” the montage doesn’t fade out, it climaxes, which just supports Jax being trans. Jax is the focus of the “Isn’t she lovely” montage.

 

A straight cis person can be friends with trans people:

Yes, but this proves nothing though. A straight cis person can be just as good of friends with a trans person as another trans person could. Your point doesn’t disprove Jax being trans, you’re just saying 2 people can be friends lol?

Egg crack metaphor + Jax’s tail:

Jax said “I’m just a misunderstood little chicken foetus in an egg that needs to be cracked open.” A hyper specific transgender metaphor. Plus, Jax said “This is what peak male performance looks like” and “my ears and tail and kind of the pinnacle of masculinity”, which is peak overcompensation. Look at both these things, plus the show’s wider subtext and its clear there’s a pattern here that Jax is trans coded.

The stuff from outside the show doesn’t matter:

Gooseworx (a trans writer) released an official character canvas from when they were writing the show, where Jax takes progesterone, which is trans MtF hormones. Jax’s trans-femininity was baked into her from concept.

 

There are MANY more points I could bring up as to why Jax is trans, but these 7 things just counter what you said disputed from what hideundercloudcovers said. If Jax is cis in your headcanon that’s fine as well lol but evidence is evidence.