I decided to use my TWSBI Vac 700R for important stuff like signatures on legal documents, etc because it should be rather easy to clean. I have a fine, a 1.1 stub, and a 1.5 stub nib for it, I'd prefer the 1.5 stub for line variation. It has been inked with Pilot Iroshizuku Tsuki-Yo before, disassembled, and cleaned with tap water until I couldn't see any ink left. I have an iron gall (R&K Salix) and a pigment ink (R&K Dokumentus dark blue), but I am not sure yet which one to use. I thought I had more time to decide, but because of unforeseen recent events, I do not have a lot of time for testing. I have no experience with permanent inks yet. I am looking for a rather easy, low maintenance experience.

Which ink should I use? Which one could go longer without cleaning? Thankful for any advice.