Something that I run into, at least in normie culture, is that writing (really) long replies to comments has a connotation of being contentious, or even hostile (example). But what if you have a lot to say? How can you say it without appearing contentious?

I'm not sure. You could try to signal friendliness by using lots of smiley faces and stuff. Or you could be explicit about it and say stuff like "no hard feelings".

Something about that feels distasteful to me though. It shouldn't need to be done.

Also, it sets a tricky precedent. If you start using smiley faces when you are trying to signal friendliness, what happens if next time you avoid the smiley faces? Does that signal contentiousness? Probably.