I found that vermouth predominant. Maybe I should try with proper grapefruit (tried with fresh spanish pomelo).

I really enjoyed this one.
I was concerned that the quality of the ingredients could make or break the result, but my first attempt turned out great.
The funkiness of the rum (Havana Club 3 Añejo), the complexity of the vermouth (Carpano Classico), and the unexpected harmony with grapefruit created an impressive balance.

I can confirm that the quality of the grapefruit juice has a disproportionate influence on the overall experience (mine was definitely subpar). I also think a mid-range rum in terms of assertiveness is probably optimal. Mine was a bit too quiet and didn't register, but an overly funky spirit would throw this drink completely out of balance.

How is it 8 in the scale of sourness when there's Rosso Vermouth + sugar syrup and not a single sour ingredient?

My grapefruit may have been on the sweeter side but a few dashes of Peychaud’s balanced things out quite well.

Indeed the grapefruit determines how the drink will turn out. I had what looked like an average Ruby Red Grapefruit. It resulted in some unusual, earthy tones, which wasn't too appealing, in my opinion. I do wonder if the sweet red vermouth contributes to that.