Hi Sarosh,
Thanks for Contacting Syncfusion support.
Please refer to the following responses for your queries:
Query #1: What is the best way to accomplish this?
We would like to inform you that the most compatible way to meet your requirement is using the custom appointment window. Since, your requirement is to change the complete layout of the appointment window. Please look into our below UG link to know about the custom window feature.
Query #2: Is it easier to just change the built-in one including adding many new fields (Dropdowns, Multiselect List, Grid etc.) and changing the layout as well?
You can also change the built-in appointment window by adding many fields and can add any controls within it. But if you wish changing the layout completely and also to add more number of additional fields, we suggest you to go with custom window design rather than the default appointment window customization. Please look into our below UG link to know about the default window customization.
Regards,
Velmurugan