I think one should have an EF and B nail. Japanese is real good for narrower than marked sizes.
F&M in regular flex..........Pelikan 200 is good for that (different size screw in nibs are not too expensive @27 or so. or a vintage Esterbrook with the right screw in nib. There are vintage '50-70-80's Sheaffers, in regular flex....in that was once the main flex of pens that were not nails.
They are sturdy enough, and make good nibs for shading inks. A nice springy ride.
Somewhere after that after the 5-6th pen, it might well be time to look for Vintage ***semi-flex. When you are not still Ham Fisted. Most of us start off that way, because of ball points or roller balls.
Real semi-flex is mostly found in Vintage German pens, but there are Pilot pens with a factory modified nib that are semi-flex.
If one alternates stub and CI from EF to BB, there are some 45 different nib widths and flexes. Chase the nib.
What nib are you missing?
What do you want the nib to do?..............that comes with experience. Better inks and papers.
Older C/C are inexpensive and just as good as new. Sooner or later you will get a P-45....nail if Stateside made, regular flex if English made.
All pen companies have their very own standards, and slop....so best to think, real skinny, skinny, middling, and wide....and don't split hairs. or you will be ......Think horseshoes when buying a nib size. (If you start with Japanese pens, you have Mark 2 calibrated eyes.....so all western nibs will be Fat. If like me, I have a Mark 1 calibrated eye, and Japanese nibs are way too narrowly marked.
Pilot makes the skinny Japanese nib, Sailor the fat ones.
With in the same company.....made in the same minute.....one nib could be a skinny M = exactly the fat F.....and at 1/1000 of an inch....a hair into tolerance, no difference can be seen.
Does the nib write well, that is the question.
No, a pen company does not want to make the exact width nib as another. Back in the One Man, One Pen days, of buying a replacement every 7-10 years when the used daily all day writing wore out a pen so one had to buy a new one...and it had to be 'up to date' ....Parker was wider than Sheaffer........and if Parker made a narrower nib.....some Parker user could make a horrible a mistake............and buy a Sheaffer.
The new numbered nibs will also have just as much slop/tolerance. A 1.0 can be 1.1 or 0.9 just as easy
Fountain pens are 1/3 nib width&flex, 1/3 paper and 1/3 ink, in that order.
I suggest getting a good to better paper for every three inks (bottles) you buy.
Cartridges are very, very expensive....get a converter or two.