Since your post was a day ago someone probably already sent you this but I figured I’d send it along anyway uwu
Hey, thanks for submitting this! I was pretty sure this was a good solution, since the cartridges are so simplistic.
A list of inks that should suffice: any ink made for brush/nib/calligraphy! Rapidograph makes excellent inks with good flow that won’t clog. Speedball is also a good ink (though a bit heavier) and Higgins should also work, though this is far too transparent for my liking. Do NOT use an acrylic based ink (liquid acrylic, etc.) because this will dry in the brush and ruin it!
(I have a Kaimei brush pen with natural hair I need to revive by giving it a good thorough rinse, as it seems to have less than ideal flow lately… it uses sumi ink, which is watersoluble.)