HN, how do you prepare a laptop for repair?
BUT this will take anyway a few hours time (imaging a disk takes time) of which some will need to be "attended".
The fastest would be as someone else suggested, to remove phisically the disk and store it safely, but you need a replacement disk (HDD or SDD) to put in the machine, otherwise the repair service won't probably be able to fully test the machine after the repair, and of course there are many laptops where you need to open the laptop to remove and replace the hard disk, so the time would be more or less the same as replacing the battery yourself.
Depending on how you have organized your data, you may be able to more simply backup, delete (securely if needed), and then restore just the data, but if the scope is "security" it is far from foolproof, and it will take anyway some time.
I believe that this (and the issues in transferring data when/if you change laptop/PC) is one of the unresolved problems of modern computing, there is so many data (scattered almost everywhere on the disk) that the only way to be sure that nothing is lost is to image the disk and - on the other hand - there is no (that I know of) way to make sure that you backed up and deleted all the data short of wiping the disk.
A good practice is to have all application data on a separate volume/partition, different from the one to which the OS is installed, but this only covers the actual documents as almost any setting will be on the OS volume anyway.
I mean, if you decide to encrypt the whole disk (like bitlocker or similar) you will need to provide the password/key to the repair shop or they might not be able otherwise to test that it is fully functional, if you have all your data on a separate volume you may well encrypt that volume only, but lots of possibly "private" data is anyway scattered on the OS volume.
After repair, restore.