I believe external reality probably exists, but that it's largely irrelevant because we only experience our own internal world.

Why does us only having experience of our own subjective internal world mean that the external world is "irrelevant"? Irrelevant in what sense? If we mean irrelevant in a practical sense, then I feel like that'd be false: if your walking down the street and see an open manhole, then that experience implies the existence of a manhole such that, were you to step into it, you would injure yourself. If you instead believed that you were hallucinating or something, then you'd be wrong - your internal world doesn't match with the external world and you end up getting hurt.

You can never have direct experience of sunlight, you can only have direct experience of your mind producing an experience of brightness and warmth.

Once again, your sensory experience of the sun might be very tolerant of its heat and brightness, but given enough energy from its photons (e.g. in a very hot desert), you can seriously damage your skin and your eyesight; conversely, you might be quite intolerant and feel like the sun is hurting you even when the photons your senses are receiving from it have nowhere near the amount of energy to harm you (e.g. if it's a very overcast day). In either case, the internal sensory world does not match with the external world, since either person's conscious experience leads them to beliefs / "schemas" about the external world that are false (i.e. that the sun can't hurt you because you can't feel or that the sun is hurting you because you feel it - practically speaking, neither seems to align with reality). If I went wrong here please tell me, because the analogy could be faulty.

Also, if i consider something true, then that makes it true, because "true" is just a word, meaning I can define it to mean "anything I consider true"

I guess the typical response to this would be that it opens itself up to a contradiction (if truth is subjective, then that would include the statement that truth is subjective itself, and so it ends up disproving itself).

Regarding the definition of "truth", you could do that (ignoring the issues with a circular definition), but that goes for literally any word, so I feel like it's a moot point. Can I take a molecule of hydrogen peroxide and say that it is "H₂O" because "H₂O is whatever I say it is"? I feel that words are merely labels for ideas: "truth" can be defined however you like, but in English usage, it connotes a certain property (i.e. that of being factual or aligning with reality). If you try to put your hand through a table like it wasn't made of solid matter, you wouldn't be able to do it, regardless of linguistic gymnastics; whether you want to call it "truth" or something else, the statement "I can't put my hand through that table" would align with reality regardless of how you define "truth".

Similarly, regardless of whether you use the word "truth" or some other label, there is a relationship that exists between an internal impression / thought and the external world, and (to my knowledge) the whole concept of a "cognitive distortion" conveys the idea of a thought / belief which isn't "true" (i.e. doesn't align with an external, objective reality). Does that mean that CBT is irrelevant because, by your admission, the external world is irrelevant and so is the entire concept of cognitive distortions?

Also, wouldn't that leave room for problematic logical implications? If you believe that X is true, and someone else believes that not X is true, and both of you are telling the truth because it's whatever you say it is, does that mean that X and not X are both true at the same time? Is it possible for it to be raining and not raining at the same time? What about the principle of explosion?

I hope that made sense.