Ok, I meanwhile managed to finally figure it out by myself, at least already for the most essential functions (Lights on/off, Set Brightness, Report actual Power Consumption). So in case that someone else is looking for the solution, here is my current setup:
.things-File:
Type switch  : SW_SHEL_3bf42c "SW_SHEL_3bf42c"  [ stateTopic="shellydimmerg3-e4b3233bf42c/status/light:0", transformationPattern="JSONPATH:$.output∩JS:shellydimmerg3_transform_light_in.js", commandTopic="shellydimmerg3-e4b3233bf42c/rpc", transformationPatternOut="JS:shellydimmerg3_transform_light_out.js"]
Type dimmer  : DI_SHEL_3bf42c "DI_SHEL_3bf42c"  [ stateTopic="shellydimmerg3-e4b3233bf42c/status/light:0", transformationPattern="JSONPATH:$.brightness", commandTopic="shellydimmerg3-e4b3233bf42c/rpc", transformationPatternOut="JS:shellydimmerg3_transform_brightness_out.js"]
Type number  : PO_SHEL_3bf42c  "PO_SHEL_3bf42c" [ stateTopic="shellydimmerg3-e4b3233bf42c/status/light:0", transformationPattern="JSONPATH:$.apower"]
and the following JS-Transformations used:
shellydimmerg3_transform_light_in.js:
(function(i){
    var status = 'OFF';
    if(i=="true")
    {status='ON'}               
    return (status);
})(input)
shellydimmerg3_transform_light_out.js:
(function(i){
var output = '{"id":0, "src":"shellyresponse", "method":"Light.Set", "params":{"id":0,"on":false}}'
    if(i=='ON')
    {
        output = '{"id":0, "src":"shellyresponse", "method":"Light.Set", "params":{"id":0,"on":true}}'
    }
   return (output);
})(input)
shellydimmerg3_transform_brightness_out.js:
(function(i){
var value = parseInt(i);
if((i>=0)&&(i<=100))
{var output = '{"id":0, "src":"shellyresponse", "method":"Light.Set", "params":{"id":0,"brightness":'+value+'}}' }
return (output);
})(input)
Hope that helps, cheers!