This is how I would do it.

For the different senders, use AutoText and AutoText fields. Again the AutoText specific to each sender is stored in a different template which can be loaded as a Global Template but which is not automatically loaded.

You can load different templates (and unload them) using the Document Template button on the Developer Tab. These global templates will only store AutoText, no macros, so they do not have to be in Trusted Locations. One template per set of sender information. I thought I had a tutorial on this, but find that I do not.

Note, if the sender information for a particular user will not change, then I would have the information for that user load automatically. I have used this in a law office that has several lawyers sharing forms. Depending on the logged in user, different global templates are loaded automatically and the information for that lawyer is inserted automatically.

Then you can use mailmerge for the different recipients. You can have all your potential recipients in one data file, or you can have different data files. If all in one data file, you can select which ones get which letter prior to the merge. You can do them one at a time, too, pulling in the information for that recipient from the data file.

Mail Merge

If you are up to doing user forms, you could easily have a userform to determine which sender data set gets used and updates the AutoText fields in the letter.

Most of my work is one of a kind based on forms. For that, I have the macro call the mailmerge function to pick a recipient. If you tell it to pick from all fields, you can just put look for something that is in the recipient's data file. If it is the right one, you cancel and work from that. If it is not, you find next. If you have a unique identifier (customer number) for each recipient, you can search for that. I then save the previewed letter as a new document in my client's file, lock the merge fields, and change it from a merge document to a normal word document. It retains the data for that customer in the fields. If doing something for multiple recipients, I would do it differently. Then I would actually perform the merge, usually to a document that can be edited.