Good job! The draft represents an interesting compromise between realism and playability. O0
I have a few remarks / ideas that might prove useful:
p.21 Titius Bode Law: This law has been proven to be nonsense. One can take a function with roughly as many variables as there are data point in a given observation/measurement and one will find a perfect match.
There is a very simple way of constructing stellar systems. The key word is Hill sphere. It it the sphere around a mass, e.g., planet, where said mass dominates the gravity field despite a nearby star. The idea is simple: Consider there is a massive planet with a wide Hill sphere. Its orbit can be set arbitrarily. Every other planetary orbit intersecting with the Hill sphere can be considered unstable because the smaller planet will get accelerated strongly everytime when passing the sphere. These planets will be pushed to another orbit or completely kicked out of the system within a few orbital periods. Hence, Hill spheres are taboo for everything but asteroids. One can play this game even further by saying that also at a distance of several Hill sphere radii the smaller planet is still permanently accelerated by the massive planet. The larger the separation between the two orbits (measured in Hill radii), the longer an orbit can be considered as stable. In addition, the shorter the orbital periods, the shorter is the stability of an orbit.
An abstract system construction would require starting with the massive planets at arbitrary orbits followed by inserting less massive planets until no more planets can be added without putting them into another planet's Hill sphere.
Sounds more complicated than it is... By the way, another stability criterion for orbits is an integer ratio between orbital periods such as between the orbits of Venus and Earth...
p.21 Water, Water Everywhere:
Have you considered to add a paragraph on water migration after planetary formation? Simulations have shown that hydrogen is driven out of the inner system rather quickly in the early phase. This is also the reason why Terrestrials are typical in the inner system while mid-distance planets gather much more gas, hence turn into gas giants. Ice giants result from gas being driven out before a full gas giant has formed, because the orbital periods in the outer system are much longer than in the intermediate range. The interesting thing about water on the Terrestrials is that it need to be transported there via comet or asteroid impacts. In simulations, this has proven to work rather good for solar-like stars. However, X-ray active stars dissociate water at much larger distances, which allows the hydrogen to be driven out of the system. In other words, the required ice comets and asteroids can not form as efficiently. Therefore, such systems will rarely feature habitable planets because the water is not only necessary for the oceans but also for oxygen enrichment in the upper crust and atmosphere. On the other hand, presuming that water is already present on a planet, the X-rays can accelerate the oxygen enrichment of the atmosphere, therefore, also accelerate the bio-cycle and evolution. This is a mechanism that could cause more habitable planets around M-dwarfs than one would expect from the first glance.
One last remark in my own interest:
p.18 quotes the IAU definition of brown dwarfs, specifically the lower mass limit of 13M_jup. Unfortunately this number is based on a "political" decision rather than actual science. There are examples of more massive planets and also of notably less massive brown dwarfs than the 13M_jup dividing line. According to simulations brown dwarf masses can go down to about 1M_jup. This does not even consider issues such as metallicity and formation history that would outright kill the idea of a single number for distinguishing planets and brown dwarfs. Of course the number is irrelevant for the game purposes but since I've been working on the brown dwarf topic for several years now, I cry a little everytime this arbitrary number is mentioned. So please, please, take this number out. :'(