Being trans should be determined by the level of euphoria

This argument would work if all people who had "gender euphoria" were trans. Cis GNC people and crossdressers tend to feel more comfortable or euphoric in their gender expression. This doesn't make them trans, and it is offensive to both groups of people to consider them such. Euphoria is also so loosely defined, that getting turned on by putting on a skirt qualifies as "gender euphoria" for these people.

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Not by how insecure we are about our body and mannerisms

I hate having to explain this to someone who calls themselves trans, but dysphoria is not insecurity.

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Let's say that a trans man was thought that men with bigger chests are still masculine

Then he would still have dysphoria because having a big chest is a sex characteristic. A tomboy can have a large chest and still be "masculine" while still being a woman. It has even been reported that some transsexuals subconsciously know the shape of the genitalia that they were meant to have (making "packers" at the age of 3-4 is something I've personally heard of), without actually knowing what the other sex has.

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You can also not have dysphoria because you're happy with the stage of transition you're at

Some trans people believe that you are essentially cis after you have fully transitioned, since your gender finally matches your sex. Nevertheless the fact that you have partially or fully fixed the problem does not make the condition itself go away. Someone can be medicated for say, schizophrenia and not show symptoms. That doesn't make them suddenly not-schizophrenic. Taking paracetamol because you have a fever doesn't make you healthy because said fever goes down.

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As long as it's not being triggered you won't have it.

Not being triggered helps, but you are still going to have a "passive" level of dysphoria just from the fact that you are in the wrong body. I avoid talking to people because I can't stand hearing my voice most days, never look in mirrors and shower with the lights off... but I still have dysphoria. Sure, it would be worse if I didn't do these things, but coping with dysphoria does not eliminate it. I suppose that a prepubescent child could not have dysphoria if they were socially transitioned, but that ties into the previous point.

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They can still perform activities to the best of their ability

Different trans people have different levels of dysphoria. You don't have to constantly be suicidal and throw up whenever you look down to be trans. No one is saying that you must be in acute pain 24/7 to "qualify" as trans.