Death's Design is interesting because of how it interacts with Double Enshroud. Because you need it for filler GCDs in the setup, you can only enter into it when you're under 8 seconds or so on the timer, or else you lose potency. That interplay itself makes it significantly better designed than most of the single issue DoTs on other jobs. That's one of the parts of the job that I don't want altered unless they want to add additional layers to it.

Your post does bring up a really simple solution for Harvest Moon that I hadn't considered, though. Just give Soul Sow a timer. When the timer elapses, it turns into Harvest Moon on its own. If you press the Soul Sow button to channel the ability as before, it accelerates the timer so that it ticks down faster. Now it has a set number of uses in a fight, but if the fight has any downtime mechanics you are rewarded for weaving channel time to generate extra uses. You could probably even attach some gauge to the action then, and perhaps allow for a Double Enshroud opener.

On the earlier point about out-of-combat actions: I think they would add a lot of additional flavor to the jobs, and would be fun as long as they don't count towards the total new action budget. They don't have to be mount effects necessarily, and you really only look to Dungeon and Dragons' cantrips for inspiration. It's also a way of getting some of the more unusual Final Fantasy spells into the game, like Float and Warp. You could even have something to evade combat, like perhaps bringing Smoke Screen back as a way of reducing aggro ranges on mobs out of combat. That could have impact on Exploratory content and Deep Dungeons.