I am loving the sb-400 for macro work. I tend to use the Laowa 85mm 5.6, which is a tiny lens. Until now I have used a Meike MK320 in Manual mode (no TTL on Z6), but I recently tried the sb-400 TTL. Very happy with it, and +/- flash exposure was easy to set up in the ‘I’ menu.
Apart from that, the 400 is so small and cute :-) Not the cheapest ancient device to buy, but well worth it.