The Void's difficulty is rather interesting because, it relies not on the fact that the mechanics themselves are hard, but because mistakes are very unforgiving.
I managed to beat the game on my 2nd playthrough (had to give up on the 1st one due to too many mistakes
) and based on this experience I can safely say that the things that make the game the hardest are:
1- Donor Glyph in general: With this I basically mean 2 things. One being the fact that I tend to fail writing the Donor glyph about 10-15% of the times, even now that I know how to draw it. I think the game should be more forgiving with this glyph in particular, given its importance.
Second reason being that accidentally coloring a tree with little lympha is a really bad idea, especially in the beginning, and unfortunately its a very common rookie mistake.
Perhaps, as it was mentioned earlier, preventing the player from doing so, similar to what happens with the sisters, is a good idea.
2- Lympha in hearts: Sisters tell you that you should fill your hearts with violet when growing gardens and gold when donating lypha to sisters when you've already spent much of it without doing so. (for example, one of the sisters tells you about violet's ''inspiration'' power when you reach 'caves', and by then you probably screwed up with Sister Death's garden and possibly others depending on which direction you took on the void)
Perhaps having a quick tutorial or having sister death, or maybe even having the colors talking to you and telling you their functions, RIGHT in the beginning of the game (maybe the 1st time you enter Death's garden), would be a good idea.
3- Predators: Sister Death tells you early in the game that Predators are an indication of the sister's realm being corrupted, and that they should be killed to prevented the anger of the brothers. I remember on my 1st playthrough, taking this TOO seriously, and literally killing all predators I saw in every chamber. This obviously caused my nerva to be lost trivially and I'd rarely have enough to grow gardens. Even if you DO know that predators are not such a threat, generally speaking the best you can do is avoid them. So perhaps Predators shouldn't be so numerous in certain areas, given the fact that it's not wise to waste color on killing them anyway (I'm particularly thinking of 'Phalanx' I think it is? One of the early game mines, although probably there are other places like that)
4- Movement: One of my newbie mistakes was generally spending 3/4 of a cycle moving from a very distant garden to another, not realizing that the amount of lympha it took me to move such distances consumed almost all the lypha I had gathered moments ago, even moreso if I had grown amber. While this may seem a good idea if you want nerva, combine this mistake with the previous 2 mistakes of killing predators and wasting excess nerva in growing gardens and opening sister's hearts, and you have a recipe for disaster.
I think a good solution would be speeding up the movement in the void. Perhaps a 50% increase.
Well, those are my two cents.