I’m no money expert, but I think as long as you identify and hold onto a niche, you’ll be doing well. Your niche right now I guess is largely tinkerers, many of whom are happy to… or want to… print their own parts.
By employing mass production you might end up with a higher gross margin, but you’re potentially changing the above proposition… perhaps to focus on more mass market, cheaper-end kits. It’s up to you to determine if you’d do well in that market.
Keeping good accounts that include an honest price on your labour and production costs will help you decide, by forcing you to make the above decisions before it’s too late. If you’re in a space where you don’t need to make those decisions yet, then don’t sweat it too hard — enjoy the niche you’ve carved out while it lasts.