Tried this out today and while its the best one I've tried so far, got some suggestions:
The manual should be included inside the software like the help pages are. It should also detail all the controls in a way that stands out from the rest of the text. It took me a lot longer than I care to admit to find out I had to click a chip and press delete on my keyboard to remove a chip.
The inputs with sliders should take keyboard input to set them, so its easier to control what they contain (such as the Input 8). By this I mean something like having a button, or being able to click on the value displayed on them and change it that way.
Multi-pin Inputs and outputs should act like their pin counter parts: for example, the Input 8 module when in a circuit opened as a block, should take an input bus to pass on to the internal circuit, as opposed to having to get rid of it and using 8 input pins or an input bus "pin" in its place, so that users can test the circuit and leave that part in place with one part, instead of having to swap them out. This would also alleviate having to have, say, an input bus pin, immediately followed by an output bus to break the incoming bus into separate wires, since you'd have a part that effectively works like this by default. I know I can make my own part to do this, but its not immediately obvious that these other parts don't. Also, an Input 8 can't take input from an input bus, and the output 8 doesn't even have an output to connect it to anything for pass through (nor does it have a bus connection), where the regular output part does. I know this is a small issue, but it is quite frustrating, especially when the in software help shows circuits being used that have only an input 8 as the initial input, but you can't also do that.
Sometimes, when reloading circuits that have an output that is feeding an output pin from its right side connection, the simulator will break either just those connections, or all connections between all parts after the initial inputs connections.
Add a drop down or some other menu option that lets you either pin commonly loaded parts as blocks, or lists a folder that contains custom parts the user has saved to quick load them. It's really slow to even hit ctrl + b and scroll through a list of my parts to reuse them, especially when I want to make a complex circuit with only my own parts (I.E. if someone was following NAND2Tetris, which I plan to with this as so far it seems the only one capable of handling this task).
Add a function to add multiple chips of the same type to the grid at once, instead of having to manually add them one at a time (especially for the input and output since they have a choice). This would really be useful for making things like an 8bit or 16bit AND chip, as you'd need to manually add 24 parts at a minimum unless you build them incrementally using your own custom chips, and that could still take longer than it should given how custom chips are currently loaded. Even adding something like Ctrl + D duplicating the currently selected parts would help a lot with this quite a bit.
Add an option to the basic logic gates to increase their inputs. For example, being able to have an AND gate with 8 inputs instead of just the 2. It's not really fun to expand these chips when you've already done it a few times and can do it in your sleep, its even worse if you're going for something like a 16:1 AND gate.
Make wires able to be recolored so you can see at a glance where things are connected to, even when all the wires are spaghetti.
Make parts that have Displays in them show their display when loaded as a block.
Speaking of displays, if you wire them inline with each other, they display things in the wrong order. For instance, if I have 2 displays hooked up, with the bus going into the left one, then from the left one into the right one, the left one displays the 1s digit, and the right one displays the 10s digit. I have to instead wire the right one first and pass the bus through from the right one to the left one - not only does this not make sense given that components are set up to construct a circuit from left to right, it also doesn't make sense if everything is going by little-endian formatting. Maybe this could be a setting in the chip to switch them between the 2 modes?
Overall, as I said, this is the best one of these I've seen yet, and I've even used logisim and its forks a lot, and Sebastian Lague's Digital logical simulator, and with these updates it would be even better. The only other issue I have is that the source is locked behind a paywall and unfortunately, no I can't afford $5 (I'd be more than happy to otherwise, but work is little right now). If the source was on github instead, I'd be more than happy to try to implement some of these things for you and pass them on to you. Definitely understand wanting to get paid for your work, you should definitely getting paid for this. Just wish there was an option for those of us that can't pay for software right now to at least contribute to the code.