One solution could be you provide regular expression instead of "/content" which serves your purpose something like restrict the selection up to level Two.(Not sure about this) Another one can be you let author select content path up to any level, but while saving it you reduce path up to level 2. This can be done by writing a script or listener for path browser field.

Content Fragment templates should not be avoided ..Make changes in the dialog of CF template and those changes will reflect in your CF Editor.For reference check WE Retail CF templates(/conf/we-retail/settings/dam/cfm/models/we-retail-store-info).