Alright I'll give my take on Ancient Runes
Why Ancient Runes?
Fundamentally it is necessary to find out what it does beyond gives bonuses to two other electives, and speculation on what it gives based upon a variety of factors. Still time to speculate!~
1) The fact that Divination, and Arithmancy skills that are given bonuses from it involve the prediction of the future through different methods imply an understanding of in it would help with finding something in Hogwarts.
2) While this isn't mechanical any room in Hogwarts that is hidden away for adventurous students is highly likely to be labeled in Ancient Ruins as the changing nature of the castle makes Qi difficult to retain meaning without the place being sealed, and linguistical drift is a thing. So from a narrative pov should Rei run into rooms with labels on them she can at least make out the generalities of the trial ahead, and make some of her own fortune be working smarter not harder by remembering the muggle common sense of reading labels.
3) Making things like pill furnaces, cauldrons, or other such things where being able to design near 100% behavior of the equipment is critical for final results. In canon we actually see the Pensieve Albus uses some type of runes, and I'd be surprised if there wasn't other such secrets in the castle that you need ancient ruins to work out how to operate without triggering whatever safety mechanisms that kick you out permanently.
4) We're basically in the best place to train the skills as there isn't any rat race over getting at least one monthly Sect point like the school year, and should we go with the intermediate trio of Transfiguration/Charms/Potions... there is a pretty straightforward* path toward the second year. Like sure there is a degree of variance depending on if we can grab Apprentice 1 Astronomy, the possibility of a dueling skill, and what we'll get from having places of power,
5) The reason Rei can train the elective skills is that the elders normally busy teaching them are free, and that won't be the case in the school year proper, but a Senior Brother/Sister seems like it should be work if she has at least a beginner skill. This presume she finishes a month, or two ahead of the July, and we want to get ahead of the third year elective fight over who earns points in those classes.
6) Building off 5) while I've already mentioned the dc reduction uh the reason for that being important is simply put we've got an unmitigated 10% chance of a boon/bane, which is an issue when you're doing 6 social actions per the two summer months with the current bane being the result of a 9. So if we spent sect points, and they offer the option of grinding divination I'd pretty seriously consider grabbing it with the dc being 2-3 lower, which is pretty massive when should we go charms.
7) Being a second year Ravenclaw with knowledge in Ancient Runes means those who either don't have Sect points to pay someone with more experience, or are trying to be frugal will come to Rei for advice. This plays well with her having connection in all four of the houses as it means they don't even need to know Rei personally, and works with the stereotypes of the houses.
There are other things I might say, but this is enough speculation to throw out for now.
*I can give a general outline of this from my pov after this vote closes if anyone is interested.