I would also think that something alike to that conical shim could work. Someone with basic CAD skills and a 3D printer could likely also make something similar without too many issues.

I'm also looking into this frame (looking for gravel bike that's a bit towards aero/road bike, with clearance for at last 40mm tyres and a reach of ~360-370mm). Can anyone post a picture or two where it's possible to see the shape of the downtube? It's super hard to make the shape out on the all the pictures I have found.

Thanks in advance for any help on that.