Are the users in-house? Macros are a great solution when the template can be distributed easily to them - not so great when the template is emailed or downloaded because the macro security can be a complicating factor.
I would explore a non-macro solution first by using linked Content Controls and put a 1 page form in front of the letter as the 'input form'. The input form can be completed, and then the user can delete that page.
To help setup the linked CCs in the file, you should use an addin template like Greg Maxey's Content Control Tools
https://gregmaxey.com/word_tip_pages...rol_tools.html