I second max dog's words on the subject of the Pilot 912 or 743 (I have the latter) with FA nib. Change the feed to the very affordable one (I call it cheap: great value!) from Flexible Nib Factory and you have a realistic flexible, modern set-up. (Regarding the FNFactory: proprietor Joey Grasty is very prompt in sending out his products!)
I have tried the Conklin Duraflex and had no luck; I changed the nib/feed unit to one from whoever it is who sells the flexy, modified Jowo nibs -- and still have had no real success. It does not come anywhere <near> the Pilot/FA combination, in my opinion! My FA nib is the larger (size 15) and this pen now affords me almost as much pleasure as any of my vintage Pelikans.
You might also consider a pen with a titanium nib (Stipula?) Although not considered a real "flex" material, the springy "feel" is pleasant on paper and I have an Omas with this nib. However, considering the sports spectacles that you may buy, made of titanium, which are so flexy and unbreakable, I fail to understand why they do not make properly flexible nibs in this lovely material...
Edited by Christopher Godfrey, 10 July 2020 - 05:45.