Damn, I somehow missed the part where they should be stored in /config/ui_lovelace_minimalist/custom_cards, my bad.
A note, it should be stated that even if you are only creating dashboards in YAML that you still need to do the integration install or the custom_cards won’t be appropriately used, and you wont have the UI_LOVELACE_MINIMALIST option in the YAML Reloading section. I didn’t add the integration at first as I figured since I wasn’t editing the in the UI editor I didn’t need to. Just something to clarify in the documentation in the future 
Thanks!