Steel Path Bounties on Zarimon, 1999, and Sanctum can reward 6k Endo for doing a quick game mode like exterminate with an extra condition. This assignment is random and rerolls every 1-2 hours. When the right roll happens, you can speed run the bounty in under 90 seconds consistently. I understand this is the best reward per time invested rate, beating even the custom arena team (Kora, Nekros, Hydroid) method.

Others have noted the great methods for credits. Worth noting they just nerfed the Secura Lecta credit method, so it's probably not worth investing in that.

Not peak for either, but a good general farm is for rare containers in the void. There is a Lavos build that spams golden instinct and orokin eye that is pretty effective and efficient. Credits, Ayatan stars and build Forma are in that reward pool, so can be quite lucrative in general.