My thoughts on
the matter.
New thoughts: all items that replicate a spell effect have charges. Ring of Invisibility? Might have 10 charges. No unlimited use items. Passive items can still exist, they're just always on, but the difference between a Potion of Cure Light Wounds and a Healing Belt is what I'm trying to address here. Make a meditation ritual that takes an hour to restore power to charged items, compared to the much longer ritual to completely uninvest your points. Until you do this, the points that had been invested in their charges are inaccessible. Figure out the cost of passive items with a spell effect as if making an item with the equivalent of Persistent Spell (which may not actually exist, but anyway). Magic items aren't limited by this; you can make an actual Ring of Invisibility that works whenever you put it on if you put the effort in.
Crafting Real Magic Items: First and foremost, it is not a downtime activity. It is a, "This is my life's work" activity. Make it take months, minimum, just for putting together an at-will cantrip. 9th level spell whenever the hell you feel like it? That's the sort of endeavor that will take the rest of your un-lifespan (silly mortals are gonna die too fast) to pull off and likely kill you. So there's that. Next, if the rest of your wealth is directly keyed off XP, then paying XP to make items is a great way of standing still. Need a way to make this work better, though, because hanging back a level does still provide permanent benefits after the first time you do it. Necessitate a feat for making magic items; doesn't have to be a different one for each type, though. Just "Craft Magic Item". Pay for mundane costs, such as the masterwork sword you're gonna make shoot lasers. No discount for restricting use to certain creatures. Crafting an intelligent magic item kills the crafter and traps their soul in the item as its new intelligence (way to dodge death, etc). Maybe make magic items immune to typical antimagic fields (the item gives the magic an anchor in reality that allows it to continue to function, but the stuff most adventurers have is really just slightly mystical and far less tightly bound, so it still gets suppressed).
What I'm ultimately aiming for is a system where the only people crafting magic items are people who either need people who aren't them to get the items, want to immortalize themselves through their work, or have some other all-consuming reason to put forth the effort. If PCs want to, they can, but chances are it shouldn't be the focus of the campaign. You may also want to include weird components that people need to quest for.