Yeah I remember was going along just fine until the first time somebody commissioned two pens from me and I realised they needed to be exactly the same dimensions. That’s when I went back to the drawing board and rethought my entire process. It was a few years ago, and my process still gets tweaked a bit now and then, but I’m happy with the consistency I achieve overall.
As for piston fillers it actually wouldn’t be that hard to produce, but the time component, both in making them an any after sales support if/when they go wrong, that deters me. If a converter breaks then you just spend 3 quid on a new one but if the integrated piston filler fails it’s very likely the whole pen is coming back to me to fix. A hassle I don’t really need. Maybe I’ll have a go as a side project though.
One thing that I do feel is a little beyond me, but I’d love to try one day, is making nibs and feeds/housings. I think about that a lot and it would take a huge amount of work. Be fun trying though.