Yep. Advanced Zakuzaku Gold Rush missions are the best I guess. Usually it's not worth it but right now, 250k gold for 20 stamina is very nice. Teh other good sources (but not farmable) are the free 150k each day and the 210k packed with 3 stamina potion on world exploration.
It's crazy to try the 4 bilions but at least, it would give me enough gold to waste on that stupid gold gacha.

High ampules are still not farmable. You can get them on world exploration (limited to 2000 of each), weekly quests, various campaigns or in the warehouse. the best source was Rose Medals but I don't know if the shop will be refilled some day. I guess we will know May 1st.
Same for Memory seeds. Not farmable. The only """farmable""" source is from event raid bosses. You can get 100 of them sometimes. And the best way to get them is luck on gogo roulette I think.
I'm not 100% sure but I don't think they have been available in memory dungeons or any other mission. Or just limited specific missions maybe. Not even sure.