Thanks for listening!

It doesn’t even apply / isn’t directed at or about the views of the “opponent”

That may be true of that statement in isolation, but it's a wider pattern in some of the things you say. For example, when the conversation recipient said this:

Also, the North pole would be the tropics as the sun would pass directly over it every day and there would be no such thing as "midnight Sun".

You responded first with:

Your reasoning is silly. The sun does what it does, and our observations of it don’t change when our perceptions of the shape of the world do. That would be crazy.

I'm sure you're not trying to misrepresent the other person's point, but responses like that strike the reader (at least me) as either a strawman, deliberate obtuseness, or a severe misunderstanding. As you said, it would be silly to claim the observations change the actual path of the sun, which is why it seems obvious to me that that's not what they're suggesting. I'm pretty certain they're trying to point out what they see as an inconsistency between what we observe ("midnight Sun") and the predictions of a flat earth model ("the sun would pass over it every day").

I think a more appropriate response is to address their perceived inconsistency. Since it sounds like you accept observations of midnight sun, I think that means explaining how a midnight sun could be consistent with a flat earth.

Edit: I see you do offer such an explanation a couple paragraphs after the above quote, so I don't think you're misunderstanding their point. But I maintain that the intervening paragraphs about what a silly reasoning it would be if... are only hurting your clarity and liable to put your conversation partner on the defensive, which I believe is (usually) the opposite of what you're actually wanting to do.

Just more food for thought, and thanks again for being open about it.