Current event maps are good for gold, plus Life Crystals are also a thing. Poking bosses and pulling equipment and basic girls gacha for vendor trash items is also a good infusion of rapid cash (a stack of 20 2* nets 5K gold, and you can burn 100K of seeds on the Gacha to get them without even noticing), items are also a good way to burn, or earn cash. Especially that burning part heh, but you can also sell them decently enough.
Best idea is to keep running event maps, and whenver you hit a critical mass of seeds, do the gacha and seel the gains.
This and - good gold use planning - If you buy all stuff for the garden, or keep upgrading your maxed out 6* with ampys each day, your cash reserves will diminish quickly (my case)