Hi Jo,
First of all, it's good to hear that you were able to get the Feature Update (version upgrade) to version 1709 installed on your device.
As to the rest of your query, this largely has to do with the storage controller that is present (chosen by the OEM) that is installed or present on the device. They do very from device to device, even within the same OEM. Any updates on/for such pertaining
to their devices still being supported for Widows 10 new version iterations would be available on their "Software and Drivers" support site page. For example, if your device treats the SD card slot as more of a SD card reader than a storage expansion
drive then attempting to use the SD card "slot" media as a memory (additional or secondary system drive space) may not be readily possible.
Do note that: Many limited disk space models that are sold by numerous manufacturers do have the option to upgrade to a larger onboard main drive for additional storage capacity, (this is not possible on all such devices). If said is indeed an option for
your device then that would, perhaps, be the better way to go, check your devices OEM's support/accessories site page(s) for more info and/or availability thereof.
That being said, I would try a new fresh SD card to try to change the save options for your apps/personal files using the Settings options in Win 10 to try again now that the installation has completed. These should be self explanatory and can be found at
Settings >System >Storage >Change were new content is saved. Again, this may or may not be possible do to the OEM's configuration of your device. For more info, perhaps concerning, follow the links below, these are a bit dated but they should still generally
apply;
https://www.windowscentral.com/how-manage-file-storage-windows-10
https://www.windowscentral.com/how-install-windows-10-apps-external-drive
https://www.laptopmag.com/articles/install-apps-sd-card-windows
Hint: It might be helpful to also scroll down to the comments section of each, if/as there are at each link, those can possibly be enlightening to the cause as well.
Good Luck and let us know how it goes,
-Richard