You are correct, appending to an existing backup set will extend the Overwrite period as from the time that the append job closes
You can either
1) Just do straight Overwrite for your INCR jobs with an OPP of 20 days (three weeks minus one day, in case the job finishes after midnight)
2) Make your INCR jobs "Append, Else Overwrite" to a media set with APP of 5 days and an OPP of 15 days
You also need to make sure that from Tools\Options\Media Management, you have selected "use overwriteable media in the target media set before scratch media" rather than "use scratch media before..." otherwise, the bkf files will not be re-used until the disk is full