I am currently setting up my new 10ii, and this involves also copying pictures from my previous phone to it. Since its a rather large number of pictures, I’d like to know if there is any better way than using plain scp, as this will go via (slow) WiFi.
E.g. is it possible to do a transfer using USB? I have a Mac, and on the device itself I’ll get a popup prompting for USB mode when I plug it, but then the device won’t appear in finder.
Yes this is possible.
About Mac I cannot say much, but on connecting the phone to Linux computer, there appears a question on the phone, media transfer or charge only. On Windows it works the same way.
If I then tap on media transfer, the phone appears on the computer like an external drive including two entries: Card + Mass storage. Then it is possible to transfer data like with any other external drive or USB stick.
Maybe check your Mac for its ability to recognize non-Mac file systems?
Well I guess when windows recognizes it, macOS should be able too (regarding filesystem).
Maybe its more of a driver issue?
SfOS uses MTP (media transfer protocol), not mass storage, so the filesystem is irrelevant…
Make sure the phone is set to either MTP or “always ask” in Settings → USB and your mac can access MTP devices (that may require a specific application, if MacOS does not support MTP natively).
Since you mentioned scp: You can also use rsync via SSH. Either WLAN or USB developer mode are suitable for that given you have enabled it.
For remote synchronization using rsync the flag --compress can save bandwidth .
One last option, and possibly faster, would be to use an SD Card.
I don’t think that changing the SD card is faster, because of the necessary removal of the SIM card and rebooting the phone. I would suggest to use the ssh option as @rozgwi mentioned or use SFTP in developer mode with SSH access on. Then, the password for the SFTP connection is the same as you set for the SSH in Settings/developer mode.
I am a Mac user.
“Android File Update” is a Mac app that I use. Seems to work good enough for me 
I don’t know haw it works on MacOS but on my linux I had to install mtp-tools to handle MTP.
Maybe you have to install https://openmtp.ganeshrvel.com/
Indeed it seems to be like macOS does not support it out of the box.
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