I am going to be real. You are right 3~7 months is a really reasonable time for a dev to add an entirely new area for the game with at least four enemies. When you think about it the actual programming of the game shouldn't take that long because 80% of what the enemies do is walk and then do an attack animation, make the attack appear under you, or shoot you from a distance. The other special ones are the ones that are Immortal and you have to hide, the weird short stack cat on the lab that keeps jumping around, the deer ghost, and Jessie, the only enemy in the game with two attacks outside bosses.
And since you already have the code ready you should be able to copy and paste it for the enemies, unless you made a spaghetti code that is a pain to work with and keeps breaking with everything you do. But even if the art should be a thing that takes a bit to do enemies have an idle, walking, attack, grab, and bad ending animation which is quite a bit, but still shouldn't take 7 months to implement just two enemies. And if the code really is a mess take those 7 months to fix it it will make updates quicker and easier, i think almost no one in the supports would be mad if the dev went and said "So this code is vile and i need some time to fix it so next updates can come out faster". Because in my mind when you open a Patreon and start receiving money it is no longer "oh this is just a funny side project i do for my bf" The moment you start taking people's money for it is more like a responsibility. They are giving you money and you should deliver it and try your best, but that again is just my opinion.
Now am i saying that the dev is lazy or that other people should stop supporting the dev? Not really, I don't know who the dev's living conditions are, maybe they are still forced to work a 9 to 5, maybe the art takes too long to make, maybe they have mental health and motivation issues, maybe the code is really shitty, or maybe they think that even receiving people money's this is still a side project for them. Should people stop supporting the dev? That is between them and the dev they all should be adults with a mind mature enough to make their own decisions, if they want to keep supporting the dev it is fine it is their money, if they want to stop and go look for other projects is totally fair too.