Usually what I have done before taking a Mac to the Apple Store for some sort of attention, usually in the past a recall/warranty replacement of a faulty screen (back in the day, the "Staingate" situation) or some other part, is to simply review my files on the drive (if I can and it's not an issue with the drive itself) and just remove the ones I feel include sensitive and personal information, such as medical history files, annual US income tax return information and files, etc. I stick those temporarily on an external drive (actually, they're also already routinely backed up on other external drives, too, but this way I know where they are when the machine returns to my custody and can then promptly put them back).
This has worked for me and provides reassurance that others are not either accidentally or purposefully looking at files that I don't want them to see. I don't care if they browse through the files in my Documents folder which are not of a personal nature (hey, they might find a good recipe or two there that I've snagged from somewhere online)!). My music and movies folders are filled with music and movies that probably a lot of other people also have, too, so no big deal there.....
My Pictures folder is filled with images that I have shot over the past several years, but most of those images are fairly innocuous, since I don't shoot photos of other people or gawd forbid, "selfies" and such of myself (how egotistical is THAT?!), and am more into capturing scenes which interest me, closeups and macro images as well as the fascinating activities of wildlife.
In summary: Before taking my machine to the Apple Store for any sort of work on their part, I simply remove anything personal or that I want to keep private before the machine is in the hands of the Apple Store employee(s) and others concerned with whatever the particular situation might be. When the machine comes home, I put back the folders and files I'd temporarily removed. This way the folks attending to whatever repair/restoration is needed on my machine can take care of that and if they happen to look at anything, see something along the way, they really won't spot anything exciting or interesting.