Even with a 122mm square taper BB and a compact double derailleur, on the smaller ring, the chain rubs against the inner side of the cage. Thanks to its plastic insert, the noise is less annoying with a Triaga derailleur than a Sora (→ metal to metal), but still.

I've seen people use a triple crankset, with ou without the external ring: Would the internal ring sit further outward than with a double? For the shifter, should I use a double or triple?

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Edit: With two, 2mm spacers for a grand total of 122+4=126mm, the chain still rubs slightly against the cage. A riding test is required to see if that's good enough. So it looks like going for a 125mm BB is a better option.

Edit: With 3+2=5mm worth of spacers for a total of 122+5=127mm, it works perfect on the bikestand. So either get a 127mm BB, or a 125mm with spacers for the flexibility.

Out of curiosity, how does that Shimano BB "for Chain Guard Installation" differ from the plain version?
Edit: On the BB-UN300-K, "drive side has a flange while non drive side is straight and has a lock nut to take up any variations in BB width." (source)