I understand. So the first chapter will be free and the subsequent ones will need to be bought. Does "Instant DRM free access to each chapter on release" means that donating the refered amount I will gain all the chapters after the are released? Donating once is enough, or one for each chapter? Sorry, but As I said i am brazilian and I don't understand many of initials out there.
About the CGs, assuming the Patreon goes well, as I believe it will, since you are already with U$ 740 and yesterday it was U$ 710 (which means you are growing), and considering you are a visual novel developer, this is much for mere 4 months of campaign (at least that I have seen), so you could always back to the first chapter and complete it with new CGs. At least I hope so. And if you are thinking this kind of thing drops fast with time (since people are just pre buying the game), you may be wrong, I am watching other two games that will be sold once released, Monster Girl Island and Malice and the Machine and both have now respectively U$ 12 000 and U$ 15 000 per month/update, and they are like that for quite some time. I think this prooves that once patrons get interest in a good game, they tends to keep supporting them. For all the projects I am watching, none of them had drop the income too much, at tops, they stagnate in a given amount. So I really have some evidence to guess that you will have enough funds to be able to revisit past chapters and improve them.
And please, do not make the chapters as Monster Girl Quest did. I loved MGQ, at least the first two parts, but the third part they completely forgot consistency and did it at anyway. For example : [Spolier] On part three the monsters couldn't attack the angels, their attacks simply pass through them, but it seems they forgot that there was a war of monsters vs angels in the past, something that would be impossible if monsters can't attack, and that isn't the only inconsistency there [/spoiler].
So my advise is: Think on this as a whole. Before release anything be sure what is planned for the later chapters is consistent with the past ones. I know this seems obvious, but Torotoro wasn't the first developer to do it, and I am sure he will not be the last. Since both are about monster girls, your game remind me of it