To your point of reducing waste, Pilot cartridges are refillable - you just need to tweeze out the little plastic biscuit, and later push it back into the refilled cartridge with the back of a chopstick or something.
I refill from Iroshizuku bottles, having found inks designed for Western pens to tend to flow poorly in Pilot F and EF nibs - a shame; I really like Herbin inks. For the actual filling, I use an adjustable automatic pipetter that I got for $30 off Amazon, before I quit doing business with them, and which can anyway be found more cheaply on eBay; the cartridges take about a milliliter, so get one whose range includes that volume. (I fill mine to 950ul, but it's not super critical.) If you haven't used a pipetter before, read the instructions carefully and practice with water first, since misuse can potentially damage the tool. But you'll get the hang of it pretty quick, and it's a lot faster and neater than syringes and dispensing needles. The tips are meant to be disposable, and it's good to have a few spares on hand in case of inadvertent bumps damaging the very fine dispensing orifice, but you can also just flush well with water after use and keep using the same tip for a long time.
Another problem is holding cartridges for refilling. I made a six-gang rack a couple years ago, but before that had reasonable results with a clothespin at the bottom of the cartridge and a bit of masking tape to keep the clothespin in place on my desk - slow, but workable.
I don't even know the last time I had to throw out a cartridge. They can crack if you have the bad habit of squeezing to push ink from a newly installed cartridge into a dry feed, so don't do that; otherwise, I've found them to hold up really well.
I appreciate your response! I am also using the same Iroshizuku bottle in both the Pilot and Lamy pens.
I'm curious about the Pilot cartridges - why do you need to replace the "little plastic biscuit"? Does it serve a purpose other than preventing spills or evaporation in unused cartridges?
No, I mean, it's just a mechanical seal, not required for the pen to function; the cartridge friction-fits the same way on the stem of the feed whether the seal is there or not.
That said, I'd still say it's worth keeping track of the seals, just so that you're not out of luck if you do find a use case that requires a sealed cartridge - carrying a couple of spares in a work bag, for example, as I do. The best way I know to keep track of them is by reinstalling them in freshly filled cartridges, but I guess I'd do that anyway since I refill in batches rather than by the each. It is a fiddly task, so I could see skipping it if it wouldn't be needed.
Side note: If you're always going to be filling and using cartridges by the each, a Pilot CON-40 or Lamy Z28 converter will likely be more convenient; these offer an inexpensive alternative to cartridges, and use a screw-piston mechanism to let you fill the pen directly from a bottle of ink - see [1] for examples and details of use. I've found converters inconvenient and messy to use, hence the preference for cartridge refilling, but they might well serve you better than refilling with even less potential waste.
(Reviewing your prior comment, I note you have a Metropolitan, which should have shipped with a press plate converter. That's definitely one to try! The page linked at [1] has info on how to use it, including how to get past the pitfalls I ran into when I tried the one that came with my first pen. Goulet says it has a higher capacity than the CON-40, too, at 0.9ml vs. 0.6ml for the CON-40. [2] [3])
I refill from Iroshizuku bottles, having found inks designed for Western pens to tend to flow poorly in Pilot F and EF nibs - a shame; I really like Herbin inks. For the actual filling, I use an adjustable automatic pipetter that I got for $30 off Amazon, before I quit doing business with them, and which can anyway be found more cheaply on eBay; the cartridges take about a milliliter, so get one whose range includes that volume. (I fill mine to 950ul, but it's not super critical.) If you haven't used a pipetter before, read the instructions carefully and practice with water first, since misuse can potentially damage the tool. But you'll get the hang of it pretty quick, and it's a lot faster and neater than syringes and dispensing needles. The tips are meant to be disposable, and it's good to have a few spares on hand in case of inadvertent bumps damaging the very fine dispensing orifice, but you can also just flush well with water after use and keep using the same tip for a long time.
Another problem is holding cartridges for refilling. I made a six-gang rack a couple years ago, but before that had reasonable results with a clothespin at the bottom of the cartridge and a bit of masking tape to keep the clothespin in place on my desk - slow, but workable.
I don't even know the last time I had to throw out a cartridge. They can crack if you have the bad habit of squeezing to push ink from a newly installed cartridge into a dry feed, so don't do that; otherwise, I've found them to hold up really well.