I tried to use shadysim (https://gitea.osmocom.org/sim-card/sim-tools) to install a STK / CAT applet on the sysmoISIM. It was not working for me, also it does not support UICC parameters and it seems that still Python 2.7 is needed.
As an alternative I have added support for gpshell (https://kaoh.github.io/globalplatform/) to use SCP02 and pass the SIM/UICC parameters.
See the new parameters "uiccSystemSpecParam" and "simSpecParam" for the "install" command https://github.com/kaoh/globalplatform/blob/master/gpshell/src/gpshell.1.md and inspect the https://github.com/kaoh/globalplatform/blob/master/gpshell/helloInstallSTK.t... example file. If "brew" ins installed then "brew install kaoh/globalplatform/globalplatform" is sufficient for the installation.
I also faced the issue that the sysmoISIM cards are only accepting SIM parameters and no UICC parameters, which is strange or even a bug since these cards are newer and are supporting the newer UICC ETSI specifications.