Talking about the skill system.
I think it is a must for light path if you are going to form a harem, but as it is still quite a long go for ~20s chapters to the end of story, so it is not the priority that we can save later to light up. For the dark path, I don't think it is useful since both ultimate can only use 1 time in a day and other skills seems not very useful, maybe you like to fuck and run or joining the gorilla group to fulfill their quest then it might become handy I think?
Next, swallowing, facials and creampies are options that based on personal interested, light up the one you like most(although it seems the scene for creampie have more times than facials and swallowing?), no judgement for that, but maybe kind of danger to take the cum selfie as it may cause consequence?
Then, the milf. As only two main Lis are milf and still unknown number for side Lis, it can wait for later to light up, or if you like Dr.riley then just light up the milf enjoyer to get more points from her. For the joint skill with creampie, potent cum, too early to have this that can save later as it just a few number of milf now, no scene for them in this time and the Lis below 40 can still pregnancy without this skills, better to save the skill points for other skills first in this stage.
For dominance, just like the above it is personal interested, light up as you like but I don't think it is useful for this early of time, only if you want to train saki in this period, but it has other skills that are more important to light up first.
Finally, mental, physical and orgasmic. I think they are the most useful one for the story now, for physical you need at least warrior(or ripped?) to start the twins path, for mental you getting points from naomi and teachers and also prevent problems caused by nina, for orgasmic you need to light up at least 1 to open the ella path(or the lesbian path in future?), so it seems these three are the priority for now.
Conclusion, in my opinion I think the ranking for the skills for now will be:
Mental/Physical/Orgasmic>Creampies/Swallowing/Facials>Light>Dominance/Milfs>Dark
