You've got the entry to the Lava Ruins thing a little messed up:
There are two conditions you must first meet: Encounter Iris for the first time on Whale Island, and defeat the boss of the Green Ruins. Then the tree will be large enough for you to insert a Tree Rune Stone, then the next day the dungeon will be open.
Strawberries only sell for 100g each and only regrow every 2 days, how are you getting 900g/d/plot since I'm getting 450? Even with the cooking that's 720g, plus you have to spend all that RP doing all the cooking. Though it does seem to have one of the best returns on cooking it, I haven't done all the math on that aspect but cooking up a million strawberries a day would tax my meager time/energy resources (plus my limit on tedium).
Cabbages sell for 680g after 8 days, but you need to keep one to remake into a seed. You still get an average of 680g/d/9-square-plot that way. Plus I just water them on the way up Whale Island which takes up very little RP for me these days.
Anyway that's one of the things I like about this game, there's not really any one way to make money, all the different economic paths are pretty well-balanced, and not so over-powered that buying all the expensive house upgrades and recipe books is trivial.
