Finish the story to make it playable all the way through. THEN go back and refine the gameplay. Notably I'd only do this for visual novels where the story is the main focus and battle mechanics/rpg elements are their to spice it up. I just don't like waiting to see what happens next while a dev feels the need to tweak the inventory.
Get to a "complete" version asap, sell your game then offer free patches. Still charge for DLC that doesn't "fix" a part of the game though.
At the very least, if a phased approach is taken, focus on reaching a distinctive stage that permits a full gameplay experience as far as progressing the story. Then, if you decide to revamp some aspect, you have a "complete" story/chapter to work within. That also gives you distinct milestones to follow. So, for example, you could spend X/X time in a year working on producing the next chapter, Y/Y in a year working on code revamp/upgrades, Z/Z in a year on bugfixes.
Personally, the things which always make or break a game/vn for me is: replayability. Having a function which allows the replay of certain scenes is one way to make a game that I have "finished" interesting to come back to. Alternatively, if there are different endings based on "paths", then it can appeal to the completionists (myself), and the folks who like trying different flavors. Being able to play the game, and revisit scenes is great too.
