What an interesting thread ^^.
But I'm not gonna read it because if I do, I'll probably delete my whole book out of shame and anxiety,

I'll read all the previous replies once I finish writing my story and only then!
My personal (very biased) opinion about Flashbacks is --
of course write one! Flashbacks is usually the part of the story that I like the most. I am just a past-fetishist, perhaps? Then again, I think my Flashbacks are crucial to the present so can't really be worked around... I have no idea about flashbacks that feel like a complete filler.
Transition? I usually establish a pattern so that it's just expected that at some point, there is bound to be a FB... But when I do firsts, they usually have some kind of a transitional lead-in from the present, like a phrase or two that will bear meaning in the FB.
I actually dislike when the authors are super afraid of flashbacks, and when they finally need one and go 50/50. As in -- they write a SUPER short flashback squeezed pathetically in between the usual present time scenes like some kind of a "yea, it happened, now MOVING ON, quick quick quick". Because -- if you're going to do that, then get rid of the flashback altogether. You can just tell this nugget of information in an Inner Monologue during present. No need to change the scene and especially time for such a small thing.
In short -- if it makes sense to do and you can actually do it without feeling ashamed or afraid and won't do a half-baked job -- then do it.
If you feel iffy about it, better not ╮ (. ❛ ᴗ ❛.) ╭.