Okay. I misunderstood in assuming you were using the Handheld Sync functionality. But Sync Library has similar shortcomings to Handheld Sync in some ways.
Note: I have always used Sync Library to keep the two laptops music folders in sync.
I see that you joined the forum in 2016. Is that when you started using MC?
I ask because the "Sync Library" only seems to work if it is used to sync the media files initially, and then it will incrementally sync new files and update the library of the Client (bedroom laptop) after that... until you reinstall Windows 10 again, at which time MC loses knowledge of what it has synchronised before.
So in 2016 the Sync Library function worked correctly, because you used it initially until the reinstall in 2017.
You have already said the same thing happened last year, in 2017, and you ended up deleting the files off the bedroom laptop and running Sync Library again, taking two days. After which MC knew what it had already synced and only copied across new files... until the Windows 10 reinstall this year.
Now you have the same problem, because Sync Library doesn't know what it has synced before, and it doesn't seem to have the capability to read what is already in the target directory.
Personally, I think Sync Library is old functionality that has just been left in MC because some people use it. I think it has been superseded by the Library (not media files) sync capabilities of Media Network. Although Sync Library is useful for the first-time setup of a second PC based on the existing setup of a first PC. I can't see any way to force Sync Library to take existing files on a second PC into account.
So frankly I wouldn't be using Sync Library,
because it doesn't work the way you want it to work. If you had read the thread I pointed to (
https://yabb.jriver.com/interact/index.php/topic,114430.0.html) you would have learned of some other options, and the pitfalls of such options. The user there selected the paid for GoodSync to synchronise files rather than MC, in a similar situation to yours, but there are free sync programs that could sync new files to the bedroom laptop, and as long as those files were all tagged correctly first, including Cover Art tags inside the file, they would import into the library on the bedroom laptop without major concerns. There are some issues with that method, and they are discussed in the thread.
Of course if the main laptop is always on (or sleeping and could be woken automatically) and available to stream to the bedroom laptop via your network, then the better solution is to use Media Network and you wouldn't need to copy the files to the bedroom laptop. Since you are using a laptop as a remote control, you must already have Media Network turned on.
So those are my recommendations; use a separate file sync tool, or use Media Network. It's up to you whether you adopt either approach.
PS: If you had restored a Library backup from the bedroom laptop after reinstalling Windows, the Sync Library function may have continued to work. If you have a copy of a MC backup from before the Windows reinstall, you could try that. Note that MC automatically makes regular backups of its library, so there should have been several on the bedroom laptop before the reinstall.
You don't say anything about saving MC library backups from either the main or bedroom laptops. I hope you aren't re-importing all your media files and losing any custom views etc. because of this reinstall.
