Thanks for the responses everyone. As for mog gardening, adamantoise rearing, and regular moghouse gardening, Ive not done any of that, and will start getting into it.
The turtles in gustav tunnel are a source, but only about 8 spawns on a 5min timer and the 10% drop rate seems accurate. Its not reliable, but if youre close to a stack and need a few, then its viable.
As far as desynthing the gullys, Ive given this a shot and came out even when compared to the cost of just buying the ingots. But now that Im thinking about it, I hadnt considered the cost of crystals, or the benefit of the mahogany lumber. It may come out slightly ahead of buying ingots. I think this may be my next path as I can maneuver the mog house to give a desynthesis bonus.
Again, I really appreciate it guys. I'll get some more numbers after doing several rounds of desynthing and report back. This may be a while, But I'll respond to this post.