https://bugs.kde.org/show_bug.cgi?id=399537
Braindump:
Sending outgoing SMS via the correct SIM card might be possible by setting an appropriate srcAddress in smsManager.sendMultipartTextMessage
https://developer.android.com/reference/android/telephony/SubscriptionManager.html looks also relevant here, in particular with SmsManager.getSmsManagerForSubscriptionId
In the SMS app we could show a virtual device per SIM card in the device combobox