I think you're right about that rabbit hole!! Thanks for the site, i'll check it out
I hope I'm not going to be guilty of leading you even further down the rabbit hole,but,on the question of inks, you may find something useful in the Diamine inks website.
Diamine are a UK company who've been making inks for more than 150-odd years. Many places stock their inks, but I usually buy direct from them. You may find some mixed cartridges or small samples.
Most pen shops sell ink samples in small phials (about 5ml). If it's not possible to fill the pen via immersing the nib in the ink, then usually the converter can be filled by immersing it separately, and then wiping it clean externally (a bit inky), or filled with a syringe or dropper in the same way as you would an empty ink cartridge.
The last lot of exotic ink samples I bought (Oster, KZW, some Japanese inks) was from Pure Pens in Wales.
You should be aware that some inks, in some pens, write either"dry" to the point where ink delivery is sparse or non-existent; or "wet", where ink gushes out onto the paper. For most normal writing purposes, the ideal is, as usual, somewhere between. In my experience, Diamine inks usually achieve this.
I don't use them myself, but there are novelty inks with glitter and other additives. They look quite pretty, but the
added solids can cause flow problems, especially with Fine nibs. I'd stick with conventional inks pro tem, if you're tempted.
I for one will be interested to hear how you get on, especially your choice of literature and any paper aids. I have a small grandson who is learning to write, and I'm keen he learns useful analogue things like how to write cursively with a fountain pen. I might well be " picking your brains" on anything I can use to achieve this objective.