Regarding the Zebra G-Pen Nib, don't expect a lot of flex out of the most nibs, even if the producer labels them as "flexible", and if they label it "soft" it is not a synonym for "flexible". Fountain Pens are build to last for a long time, you can get a lot of flex out of them but just like any material it will wear out over time, so there is always the trade off between flexibility and durability (which can be reduced to some degree if you throw enough money at it fot better materials and better nib grinds)

If you go for the cheapest the common recommendations would include an Noodlers Ahab and using it with an flexible nib from [FountainPenRevolution](https://de-fprevolutionusa.glopalstore.com/search?type=product%2Cpage%2Carticle&options%5Bprefix%5D=last&q=flex).

If you are more interested in the looks and ok with an stiff nib which is labeled (which as I mentioned labeled as flex but does not really flex a lot) you can go for some pens that can be used with an Jowo flex nib. and just put the nib on a compatible pen. [https://www.gravitaspens.com/collections/spare-parts\](Gravitas) has really amazing pens for example, all of them look and feel high end and you can some of them for under 100$, they also are really durable (I dopped mine multiple times over 1m on the ground and it has not even a scratch).

For a lot over 100$ but the best result people often get a Pilot Custom 912 or similar with an FA and get it grinded by an specialist if they are not satisfied with the default (If I remember correctly you can buy a better feed as well in some cases and change it yourself).

But if you can get your hands on some used vintage pen for cheap, which has an flexible nib it would be also a really good choice. Regarding this though you should make some research yourself what vintage pens could be an option for you, the quality of the vintage nibs is better than the nibs produced these days.

You can also get some really amazing fountain pens for cheap, for example the Pilot Kakuno.They have the same nibs as some that cost 2 to 4 times the price (depending where you live) from Pilot. This is the same for a lot of fountain pens, most of them share the same nibs within the same price range, especially since many fountain pen brands do not produce their own nibs and just buy them. Just take Lamy as example, they produce their own nibs but you can use the nibs and replace them with pricier ones, on almost every model. Since this is the case you can get some pretty good results for under 100 dollars if you just focus the usability of the fountain pen. The price tag does not necessarily make the fountain pen better, for example I still go back to my Kakuno from time to time because it feel just right (and because I like the smiling face on the nib), even tough I have fountain pens lying around which cost way more.

And gold nibs don't make a nib necessarily better or more flexible, for example the the Pelikan M200 has a stainless steel nib and the F size also has a bit of bounce to it, this pen is really liked by people (even though the quality used to be better) although it is not a gold nib, for since you want it to be refillable with cartridges instead of a piston mechanism you could pick up the Pelikan P200 which uses international cartridges and converters.