Ok, I found the way through. I was looking for a way to get the SuShell GUI to grant su-access to termux. It turns out that one just has to start the "sushell" (in /vendor/bin/) executable from the remote "adb shell".
(Of some reason, this executable does not work from termux in Emteria 13.4.0... In Emteria 11.11.0 I reckon it lived in /system/xbin/sushell and did in fact work from termux)