Oh right, that was not the whole Makefile, only the part that caused problems. As @fruitcantfly@programming.dev pointed out, it breaks down with more SOURCE files and populates the newly created directories with only the first SOURCE file. After some more poking and reading I found a method that works by using define:

define rule
$(DESTINATION)/$(shell echo $(notdir $(1)) | sed -E 's/^([0-9]{4})-([0-9]{2})-([0-9]{2})-(.*)\.md$$/\1\/\2\/\3\/\4.md/'): $(1) | $(DESTINATION)/$(shell echo $(notdir $(1)) | sed -E 's/^([0-9]{4})-([0-9]{2})-([0-9]{2})-(.*)\.md$$/\1\/\2\/\3/')
	/bin/bash ./myscript.sh $$< > $$@
endef

$(foreach f,$(SOURCE),$(eval $(call rule,$(f))))

I feel like this is an abuse of Make, but it works. When is it a good idea to delegate the building process to other tools?

Sketchfab is well-known in 3D rendering and gamedev communities. I don’t think it has many print-friendly models.

I use KDE Connect. Works well with Tailscale when I’m outside my home network.