If I were you I would buy bottled inks, and use the converter that came with your pen.
Why?
Because:
ink bought in bottles costs only about 1/5-to-1/4 of the price of the same amount of ink bought in cartridges;
buying ink in bottles frees you from ‘vendor lock-in’;
it enables you to use far more colours/types of inks than the now rather-limited range that Parker still makes;
Also, and perhaps even more importantly:
filling your pen by ‘sucking ink up’ with the converter actually helps to keep your pen’s feed clean!
This is because doing that means that are moving ink in both directions through the feed, and (in comparison to the rate at which it flows while you are writing), you are moving it fairly rapidly.
This helps to prevent any ink from drying-out inside the feed, and/or deposits potentially building-up inside the feed’s channels/fins, and thereby restricting ink flow.
The converter is also a brilliant tool if you wish to flush/clean-out your pen thoroughly, e.g. when changing types/colours of inks.
This is particularly true for pens like the Duofold, whose nib/feed assembly is not as easy to remove as the nib/feed unit of e.g. a Pelikan piston-fill pen, or a Parker Sonnet. Those pens’ nib/feed units can be unscrewed manually.
For clarity: you don’t need to ‘obsessively’ clean-out your pen all the time, let alone dismantle it to do so!
You might want to just run cold tap water into the top of its grip section to ‘flush’ or ‘rinse’ it, once every few months. This is what manufacturers used to recommend for pens in continual use.
OK, I flush my pens every time they run out of ink.
But that is because I like to ‘rotate’ between my various different pens, and between different colours (& types) of inks.
I use dye-based inks, acidic inks, alkaline inks, very-acidic iron-gall inks, and I use some pigment-based inks.
Several of these different types of inks do not ‘play nice’ with each other!
Not only does any individual pen that I own therefore tend to sit unused for months at a time, but I want (or rather, I need) to avoid the prospect of any chemical reactions occurring inside any one of my pens between any residue from its last fill of ink, and whatever I eventually put into it next time.