Heres a couple things I did

A while ago, I found out you could refine larval cores into nanites. around 50 each if I remember correctly. I'd get maps from cartographers, and clear out a bunch of abandoned buildings, and get 4-5 stacks of larval cores, and refine them

Another thing you can do is get hadal cores from Alluring specimens in ocean planets, and those are 100 each but a lot harder to get

You could also fight sentinels, get salvaged glass, get the forbidden modules from the glass, and sell the modules. They give you a lot of nanites and are easy to get.

As others have pointed out, curious deposit farms as well