What do you disagree with? You didn’t mention that this is your usecase.
The short answer is that SHA256 is considered one of the strongest cryptographic hash functions and you don’t have to worry that it’s possible to calculate file contents from the hashes. There isn’t anything meaningful anybody could do with the hashes besides infecting the files in the drive. The biggest danger is always plugging your USB drives into untrusted and possibly infected computers and plugging it back into yours.
In one post you stated explicitly:
For example if root control is gained over a running server Syncthing the syncthing data folder (and the corresponding file block hashes) would be in the hands of an adversary while synced files on an unmounted encrypted partition or USB drive would be secure in themselves.
However this doesn’t make much of a difference from a security perspective. Once the host computer you plugged the USB drive in is infected you have a problem. At least if you ever plan to re-use the USB drive, as it may very well be infected with malware which you can’t remove by deleting the files or even by replacing the HDD inside the USB enclosure. If you want to know more information on such threats, you can search for badusb.
A similar threat is that one of the computers you are syncing files with is/was infected with malware at any point in time before or during you are syncing with it. The malware could inject exploit code into one or more of the files you are syncing with such the infected device. So the next time your operating system scans your filesystem to update it’s search cache or once you open the file, your computer can become infected as well.
I am not saying that this is incredibly likely (it isn’t, because Syncthing isn’t very well known and popular). But that’s what’s technically possible. From a security perspective the danger that existing weaknesses in your operating system and other software are exploited is much bigger than that somebody will be able and willing to spend the time and money to try and attack the pretty strong authentication and encryption syncthing uses. Additionally Syncthing is written in Go, which is a memory safe language. This means it makes it impossible for developers to make the kind of mistakes which most commonly lead to the software beeing vulnerable to exploits.
If I wouldn’t think that Syncthing was the safest application to sync files, I certainly wouldn’t use it myself
Regarding your question:
If your relatives can’t decrypt the LUKS/Truecrypt/Veracrypt container (which also needs to have the syncthing config directory) then only the scenario of malware in the USB controller and in files on the unencrypted partitions applies to you.
If your relatives can decrypt it, they can also delete files from your directory so that the deletes will propagate to your syncthing devices once you plug it into your own computer. Maybe they could also modify the hashes, but as I’m not sure in which situations syncthing recalculates them, I can’t exactly tell you if this would have any impact at all. My guess would be that the receiving computer notices the hash mismatch and at worst you will get some unsyncable files. If ST acts smart and recalculates them on the sending side, nothing happens at all.