There's absolutely nothing about Flandre in SSiB, alas. ...or almost any print work besides her BAiJR interview and PMiSS article. You can see a little bit of the going ons for the SDM in various print works including chapters where the three fairies infiltrate it etc. From what I can tell, they're constantly finding excuses to throw private parties (stated in PMiSS but also kinda shows up in SSiB and one of the Three Fairies chapters) and otherwise just tagging along after whatever crazy scheme Remilia's cooked up lately (the rocket, temporary sea in SSiB, her pet chucacabra in FS, forcing Sakuya to drag people over to conduct an "investigation" into the odd weather in SWR, soy bean anti-oni festival... though that last one was Patchouli's. Patchouli also made a bunch of crop circles in the garden once. ...okay, whatever crazy scheme Remilia or Patchouli has cooked up)
When Patchouli or Remilia don't have a crazy scheme going on at the moment, life is kinda boring as far as I'm aware. Meiling's bio says that every day life can get boring so she subliminally cooked up her adventure in Hisoutensoku, Remilia's more than happy to find an excuse to do SOMETHING when Aya drops by in SWR, Patchouli's been depicted on multiple occassions as just reading books in her library (on one occassion with Marisa n a Three Fairies work, on another with her getting really mad at Marisa after a theft by Marisa results in a book avalanche), the three fairies once infiltrated the mansion at a non-party time and everything was pretty quiet (IE, boring?) with Remilia sleeping in the day and Meiling looking out and Sakuya cooking, etc. Although making life not-boring is probably a primary reason why Remilia keeps coming up with wierd fleeting schemes in the first place. The fairy maids seem to mostly just take this in panic'd efforts to keep up or in stride (SSiB) while sometimes grumbling about it (an early Three Fairies chapter. Possibly a case of early-installment-wierdness, although I do wish there were other depictions of Remilia's maids showing capacity to socialize and live and think about life)
Again, there's sadly a noticeable lack of Flandre in all of this.
I have regrettably not read most of the print works. Is there somewhere that I can read translations?
The touhou wiki (en.touhouwiki.com) has pretty much everything besides the latest FS and WaHH chapters (although there might be a few FS and WaHH chapters that you can only find on other manga sites, and the latest chapters aren't available). The SDM always gets a story arc/chapter or several in almost every print work. ZUN seems to like them (well, he did say in an early afterword we could expect to see them a lot in print works). ....again, not very much (...none, really) Flandre alas.