This looks like a good guide for my situation:
I did find “Max Concurrent Incoming Request Ki B” under Advanced → Options which I guess is the same as the mentioned maxConcurrentIncomingRequestKiB and set it to 32768.
I guess for the other’s I’ll have to find where the Debian packager has put the config file in the filesystem..
UPDATE: Is this the right file? /home/syncthing/.config/syncthing/config.xml
UPDATE2: I believe it is, since it contains this which is not the default value:
<maxConcurrentIncomingRequestKiB>32768</maxConcurrentIncomingRequestKiB>