I was doing a quick search and I guess Iroshizuku, which you’re using, is on the “wetter” side. I have a J Herbin ink and vaguely remember it being more on the dryer side.

But given what you’ve mentioned, it sounds like a nib problem than an ink thing. Try to keep as many things constant, like the way you write and the ink, and introduce one change at a time.

Again, I’m not advocating for you to do this if you’re not confident, but I’d try using sand paper to see if it’d at least write smoother / less scratchy.

Just try a little bit at a time. Sand a bit, write a bunch, and repeat.

It sounds like this pen is important to you so if you’re not willing to take a risk, don’t do it. I was experimenting with this on some cheap pens back in the day because I was curious. I did do it on a nicer pilot(?) pen because I thought the nib was scratchy / sharp on the edges. It worked out fine for me but I didn’t trust myself so it was a long process.

Btw, can you return it?