I think a lot of intps either avoid dealing with people altogether, are abrasively blunt, or over accommodate to the other person's preference to avoid hassle. I think it's possible to be quite authentic and honest without being brutal if having an authentic relationship matters.
The intp I know best has made huge efforts in that way. He has made it a priority to see what matters most to different sorts of his closest friends and customize a bit for them. I'm sure with me there are things he thinks but chooses not to verbalize. However, we've also managed to talk some about our varying communication needs as well as misunderstandings that have come up, and it has built a lot of trust for me towards him that if I bring something up, we'll make it safely to the other side.
On two occasions, he's brought up things I've done that have bothered him, and in a strange sort of way, that too built some trust in my mind that he wasn't just pretending things were okay when they were not and that he trusted me to respond openly. I know he values his internal privacy and also doesn't want to hurt my feelings, so there's probably stuff I'm not aware of, but I think he does try to be honest with me as much as possible.
Probably as an infj, what's helped me most is the willingness to explain or explore how he operates, as I get much less panicked or upset when I have context and information to work with. As a result, I don't imagine that it's the end of our friendship or that he's upset when he disappears for a day or two after communicating every day. I still get a bit uncomfortable if the pattern of our communication inexplicably changes, but usually I'm fine with how things are as long as I feel like I'm in the loop.
With nfjs, they get a bit hurt or hovery when not granted enough information, but if they understand the why of how something works, they are actually pretty flexible. I think that's maybe hard for intps to get used to, but if they value the connection and like some space, but dislike emotional kerfuffle, giving even insignificant details for the nfj to hold onto allows them to feel like they have a place in your world and that you value them.
For me, I also find it easier to give more space if I know that they will eventually make it a priority to reconnect with me. As long as I know that I matter to the other person, I am willing to adapt to their favoured mode of interacting much more easily.
Even if something isn't my favourite, I can deal with it if the other person is keeping my needs in mind from time to time as well. I'd actually prefer for them to be genuinely themselves and occasionally hurt my feelings, rather than try to accommodate for me and find the relationship rather taxing or unpleasant as a result.
I don't know if this is true of all nfjs, but for me, I mostly get hurt when I am seeking reassurance or practical information and it FEELS like it is being withheld.
In any case, hopefully this gives you something to work with....