On my current phone I have a 1TB SD card that holds copies of documents from my computer. To sych this, I need to remove the card and put it in a reader attrached to the computer. Any other way is terribly slow.
Can I mount the SD card directly as an external disc under eos, via the USB cable?
You can access the SD card from a computer via a USB cable, yes (on a Mac, you probably need to install software like OpenMTP to be able to do this, and it will not be the same as an external disc or external drive). But from my experience (on a Mac) this is a slow process, too – and it is unstable (often disconnects out of the blue and requires re-connection). I still do it via USB all the time because I don’t want to wear out the SDXC card from frequently removing it from the phone.
An alternative option would be a USB-C memory stick for transfer between computer and phone (SDXC card in the phone).
Yes, I can confirm that the data transfer via USB Cable is very slow and depends also which phone you use. But in my experience the transfer is always stable. I noticed earlier some problems with some cheap USB cables. So with original Fairphone cable there was never a problem. 
To transfer via USB memory stick is a good idea. I use a so called “DUO” Stick which has USB-C port on one side and USB-A port on the other side. I let the stick format at phone, so the folder structure is also as on phone memory. Also good for backup the SD-Card, cause I’ve had defective SD-Cards ones before.
If command line tools are an option, here I described how I sync a folder on my Fairphone with my PC (the photo folder in my case but could be used for anything):
Edit: my use case transfers files from the internal memory, not SD. It is possible to have termux read/write to a termux-specific folder on the SD card but not the SD card contents “outside”.
So sorry, no option, just tried it. 