@Valdair Try Procolor or Somiko (Young Profit) for steel nib and Sapporo for 14k gold nib. For me the best one is Sapporo MF, and F and M are also good. Their nibs are a bit on a dry side, but it can be tuned. You may also try 21k, but it's much more expensive considering the difference with 14k in writing experience.
It's worth noting that almost all Sailor nibs are not flexy at all. Even Pilot Custom 74 F gives more flex.