As others said, Synapse can sometimes be very resource-hungry. It might be worth giving a try to Conduit and Dendrite, which are alternative Matrix server implementations and especially Conduit seems to be focused to by lightweight. Although I do not have any personal experience with them and it seems that they are most likely a lot less mature than Synapse at the moment.
Give testdisk a go, see for example this tutorial. It is a terminal utility, so it might take some time to get used to it. But no one can guarantee that it will successfully recover anything, the deleted files stay on the disk only as long as they are not overwritten.
Do you have any idea why the files disappeared after reboot? One thing that comes to mind is that they might have been saved in /tmp, in that case I believe recovery would not be possible.
Regarding to which files you should recover, try all of them and see if you have any luck.
Good luck with recovering the files!
I did not know any of the programs mentioned in the post, but some of them seem really nice. Can someone who thinks aliases are a better solution please explain why they think so and what is their advantage over these projects? Do they have any pitfalls that you are aware of?
I believe that if I use a command sparsely enough, I will forget the created alias name just a few days later than the actual command.
Also see pairdrop, it is a snapdrop fork that allows connecting devices on different networks using a numeric code and has other improvements.
So far I have had good experience with kopia. But it is definitly less battle-tested than the other alternatives and I do not use it for too critical stuff yet.
From my understanding, files cannot be directly stored only in a timeshift snapshot -- they must be first stored on the disk and only then timeshift can make a backup inside the snapshot. But I have never used timeshift myself, maybe I just completely misunderstand how it works.
Sorry, can't resist: it would not be 6.5 % of Linux users, it would be 6.5 % overall. That would mean about 54.8 % of Linux users.