Examples of Bots created with JavScript. These provide a very granular of customization of GUI as well as functionality.
Can be done with any platform e.g native Javascript / ReactJS / PHP etc. Examples of customizations:
User based functionality: usage limits / doc. size limit / restrictions for doc upload / restrictions around URL upload./ choice of models
Choice of models beyond OpenAI
Possibilites are limitless. Pretty much whatever you can imagine can be done.
 
However, this needs developer resource as well as ability to manage a mini IT Setup, either inhouse or outsourced. Higher time to deploy and higher initial cost. But likely lower operating costs by utilizing lower cost / free models, customizing RAG pipeline etc.