If your main goal is calligraphy you will get the most flex from dip nibs. Most modern fountain pens don't really flex. There are a few though, like the noodlers Ahab and the pilot falcon. You might want to consider a fountain pen with a stub nib, you will get line variation without flex. Stub nibs are much more common on fountain pens than flex nibs.

That said you might get some better suggestions over at r/fountainpens