I can confirm I've solved this issue by changing the integrated cockpit with another brand.
The issue was 100% related to the stock C-ring that came with the Tantan aero cockpit.
Thanks again to you guys who gave me the correct advice about it.

For the new one, I didn't go with the Lexon gravel bars (which are popular around here) since I didn't like the huge flare.
Instead I went for the Rpantahi (still flared, but just a couple CMs) since it got a lot of positive feedback from other riders in my cycling club.
The price was quite good (around 90 euro) the quality imo was slightly better than the Tantan one, and so far it has proven to be rock solid even on light MTB routes.

The Rpantahi came with two different C-rings, one was plastic with a few holes and one was made out of metal with the classic C shape. I've risked it with the metallic one, but I guess the quality on this one is higher since this time I had no tolerance issues.

I've had it for more than a month, but I only wanted to give a feedback after I was sure about its quality.

Could you post a link to the C rings?

How many mounting holes are there in the frame?
What should be used to cover them? What will be the lightest?
Do you have any reliable links?

I run a Riro carbon crank set, shimano sized spindle, 46T with a 2mm MUQZI spacer and still have enough clearance. That would be the absoulute max. I would run tbh.

Anyway, does someone managed to mount a Tailfin rack on this frame, using the quick release for the extender thru axle? I'm not familiar with this system, if the frame has a UDH.

I'm not sure if I understand correctly. My English isn't very good.
I'm ordering parts and don't want to make a mistake.
Will it fit a maximum of 48T or 46T?

https://pl.aliexpress.com/item/1005008108016818.html?spm=a2g0o.cart.0.0.4825452czQH8Wh&mp=1&pdp_npi=5%40dis%21PLN%21PLN%20268.00%21PLN%20255.59%21%21PLN%20255.59%21%21%21%40211b61a417582505566186458ec03e%2112000043803181878%21ct%21PL%212077547818%21%211%210&gatewayAdapt=glo2pol

So same saddle height although you are 4-5cm shorter than me. To me the 58 feels perfect. For you the 56 might be better because you have a shorter upper body (not considering arm length though) but it means that you have a higher saddle elevation which not everybody likes or you have to use more spacers. Anyway, the 54 cannot be the right choice

Thank you. This is exactly the answer I was expecting.
Factual and concrete.

Hi everyone.

I am in the process of purchasing a GR201 frame.

Could anyone advise me on what size to choose?

I am 184 cm tall.

My inside leg measurement is 90 cm.

The saddle height from the bottom bracket is 80 cm.

54 with a 110 mm stem or 56 with a 90 mm stem?