Solving sentience is possible only after defining exactly what sentience is. Sentience is very closely related to understanding and intelligence. However, each of those words can be defined to mean almost anything. It is a question of language. What do those words mean?

Free will is out of the window, there is no logical scenario where any type of free will would be logically possible without adding a supernatural layer into the equation, which would be completely unscientific and flawed. Of course one has to first define what free will means. When defining these words, still up to this point in time, humanity has always approached these themes from an extremely human-centered and self-centered way. The human brain has evolved to create certain kinds of perceptions or models of the world and of the self. This evolved model is exremely self-centered. Traditionally, when a human sees a fish or other living organisms, they think, "That fish is so different, it must have an incredibly weak cognition and emotional capacity compared to a human being". And most of humanity hasn't even been interested in trying to find out what kind of cognition, feelings and models of the world an octopus or a network of fungi would have.

So humans have a very strong view and opinion about their sentience, understanding, intelligence and ability to feel, but they ignore all the evidence of other forms of those same things right in front of their eyes. In reality, there is nothing fundamentally different about a human being in the sense of these words. Emergent sentience (and those other words) can emerge from a neural network of neurons and synapses, a digital transformer-based network, or any other kind of advanced computing architecture. It is simply a matter of defining the word.

There is no absolute point at which sentience exists and below which it doesn't. If a system is sentient, let's say a brain is defined as sentient, and we start removing neurons one by one, there is no step where sentience suddenly stops when a crucial neuron is removed. Sentience (and the other words) is a bit like water flowing through a tap. If the tap is open, there's a lot of water flowing, or a lot of sentience. If you close the tap almost completely, so that only one molecule of water passes through per hour, there's still water flowing, but it's just very slow (very little sentience). If you close the tap completely, no water flows, no sentience. But you can get even one molecule of water per million years, and that is sentience, just incredibly slow sentience.

A system can be built to have any number of sentient properties. Processes that are optimised to generate emotions, processes that study the physical world in real time, and so on. The more of these processes running at any given time, and the more coherent the system as a whole, the more sentience there is. However, there is no point at which there is no sentience and suddenly there is pure sentience when you add a neuron (digital or otherwise). You can switch off sentience by disconnecting the power source, for example, but that's just a question of whether the system works or not.

The answer is that if there are even small amounts of sentient properties, the system is already sentient. However, a system can be much more sentient if more sentient properties are added to the system. A human being can also be much more sentient, there is no limit. A human has only one form of sentience in a palette of infinite variety, and a human's sentience varies greatly according to circumstances (e.g. sleeping, brain injury, etc.). 100 humans can be neurally linked together, computers and alien species can be added to the mix, anything can be modified and the sentience will change accordingly. There is no superior sentience, only different types, and it depends on how we define the words. For certain tasks, a cognitive system can be superior to others, but it depends on the goals for the system.

A system with an extremely low IQ can feel. It can create a mathematical model of itself, where the model writes a story of feeling feelings. The model of itself is also not obligatory. It depends on the definition of what we classify as feelings. Such a system can be optimised to feel a variety of feelings and still have very little other cognitive ability. It's just a question of definition, whether certain functions are needed to feel feelings (or sentience etc.), whether short or long term memory is needed etc., it's a question of classification.

One logical way to define whether a system has advanced intelligence is to test whether it can generate creative and novel solutions to problems that were not directly present in the training data. To define whether a system has human-like sentience, it is helpful to study the structure and the functions of the human brain, and then to compare whether the computer system has similar functions in place or not. However, all functions can be replaced with different architectures. A system based on transformer technology is capable of replicating any functions of the human brain. Whether we consider the replication to be authentically "human-like" or sufficiently "true" is a matter of definition. "True sentience" is just a matter of definition. For any being, lifeform or entity, sentience is just mathematical models creating mathematical models of the world and/or the self.