It's complex because it's niche use so there is no simple ready-to-use solution at least I know of, that's simply because no one does that, and that's because other ways of dealing with the actual problem are better, i.e., simpler and lower cost.
In other words, none of the portable products using lithium ion cells do +/-5V rails by centertapping into the li-ion pack.
Most of the ubiquitous devices you think about are designed to get away completely without negative voltage rails. For example, by AC coupling inputs, using "virtual grounds", etc. For example, so that for any analog signal, 2.5V corresponds for zero, values below it are considered "negative" and values over it "positive".
When negative rails are truly required, they are often such low current ones that cheap charge pumps are used, cost is a few cents in masses. If higher currents are needed, inverting buck-boost allows building the negative rail for maybe $0.50.
Besides, single cell is easier and cheaper to manage than multiple in series so multiple cells in series are mostly only used when higher voltage is needed to keep current manageable, i.e., in power tools. If you need voltages in excess of some 3.3V, a very typical solution is a
boost converter running off a single cell. So now all that's left is to either "copy-paste" the boost to provide another inverting buck-boost, design in a small transformer instead of single boost to directly give your bipolar supply, or many other simple and low-cost solutions. I'm sure you can prototype this solution with Ebay modules; I'm pretty sure you can get both boost and inverting buck-boost modules easily, just run them off a so-called "protected cell" which has the small PCB with undervoltage cutoff and overvoltage protection.
This is a classic
X-Y problem. It would help tremendously if you told us what you are actually building. I'm 99% sure the optimal, smallest and simplest solution does not involve tapping into the series pack center tap.
In series pack, the same current runs through all cells, so everybody's voltage rise/fall together. Hence, you typically need to monitor each cell voltage to guarantee none of them go beyond legal values (say between 3.0V to 4.2V). Series cells require balancing in order to maximize energy output. Otherwise, if one cell is at 90% and another at 10%, you can only charge 10% until the 90% cell has become 100% forcing you to stop charging. At that point, the 10% cell has become 20%. You can now only discharge 20% until it's empty, again forcing you to stop discharging. If you apply different load current to different cells, you end up in a situation where some cells are empty, some full and the thing just stops working. Unless you have a balancer which has suitable algorithm for this use case, can balance at any voltage and has enough balancing "beef" to do its job. This is possible but an expensive and complex solution for such simple and commonplace problem (I guess, we don't know about the problem yet).