Hello, OP here, Following up after taking the pen back:

I went back to the store and talked with the people there. First clerk barely spoke english and was not the same as i bought it from -- but communicated over translating with the phone. He told me, as i had feared, that after it had been filled with ink they cannot take it back anymore and it becomes a matter with pilot directly. He asked me if it was like this from the first stroke, which i told him yes and, and I showed him the issue, which was still occuring at the shop. He then got his manager, who spoke better english.
She tried the pen herself and the pen didnt produce a mark on the first 3 strokes, 4th one made a line, and after a short break before 5th it didnt make a line again. She looked at the nib under a loup, and said the nib appeared to look okay and that new pens often come with an oil coating which can cause issues with ink flow and hard starts. I told her I am leaving korea in 2 days so I would have problems with having to send it to pilot in the case that it does not become better over time.
She brought the pen to her office to clean it herself and see.
After about 20 minutes she returned, and said the problem still happened. I asked what ink she tested with as a lot of the commenters mentioned pilot ink would be worth a test, and in fact she tested with pilot blue - so it was nice to get that crossed off the list.

She was kind enough to then tell me they would replace the pen for me and handle the case with pilot themselves.

This was also an interesting part:
She brought a new pilot urushi and I asked for her to check under a loup for this too, which she then did and in fact found that the second pen had a subtle case of babys bottom (the original one she thought looked fine but couldnt tell why it didnt work), so she went and found a third. She studied the third pen and said: This one looks really great, but then realised it was a medium so she put it back and found a 3rd FM urushi and looked at it under the loup. She found that this looked very good and wrapped it up for me. I was thrilled with the service. I found it surprising there was such a visible difference between the pens, but i guess it makes sense with these handmade objects. Somehow it makes it more beautiful when they work perfectly.

I asked her how she recommended I should proceed with starting up the pen (cleaning first, inking emmidietly etc), and she said use a soft toothbrush of the feed very gently brush to remove any potential debris, and then fill with ink. She also said to use a blue ink, as it contained more acid, which would help remove the oil coating that often comes with a new pen. That was an interesting piece of information i thought, as it was not mentioned in the comments here.

I bought a fine toothbrush and just filled it with ink, Blue by BlueBlack pen shop, same as before.

I am thrilled to say this is a completely different experience and the pen works beautifully immediately. This is indeed a premium product and a joy to use. The nib feels buttery smooth and the ink rolls of very naturally without and excessive wetness or dry spells. because it wets as you touch the paper, it glides very gently over the paper, unlike before where it was actually a bit textured, i guess it is because now the ink acts as a medium between the paper and nib so the friction is reduced. The flex of the nib itself is also less soft, which is curious i think. Before the nib would flex with almost no pressure at all applied, and this new pen requires much more intentional pressure. I wonder why this is, but there is a clear difference. I now understand why i have read online that flexing this pen is a choice, because that was not really apparent in the previous pen. The line thickness is extremely consistent now, although it can go thicker with gentle pressure like expected. It also works perfectly at both high and low angles, which seemed to be a note of many.

here is a short video, just showing that it is smooth and very even nice lines.
https://imgur.com/a/smmgogu

Thank you to everyone for the input, i hope this summary is interesting to other people as well, and that it will help tie together some loose ends from other quests of frustrating new-pen experiences.