I agree, the ATA lists are a great place to start for worldwide stamp issues. They will help you focus on how broad or narrow you wish to collect and what tangents to include.
The mostly-unlisted challenge beyond the stamps is how far to go into related postal collectibles - if that appeals to you.
Postal history of pharmacy company "corner cards", their return addresses, which are often illustrated:
Some will have enclosed letterheads:
Promotional slogans in meters and postal cancels:
This is the only U.S. slogan of any sort I know of in the circular dial:
There are likely about 100 cachets for the pharmacy stamp, plus posters, publicity, etc. Here are three cachets on First Day Covers with a possible tangential interest:
Postcards (Pharmacy class at Tri-State College, Angola, IN and pharmacy interior in Clinton, IN)
Another postcard, a woman pharmacist/chemist distilling the heart of a man:
Trade card tangent, front and back:
Here are two older ATA handbooks from 1969 and 1971:
Narcotic tax stamps, both used by Eli Lilly & Co.
"Special Tax Stamp" issued to a pharmacy:
Prohibition-era prescription form for "medicinal liquor":
And possibly even medical marijuana tax stamps and "Cinderella" seals and labels from conventions and the like.
Topicals are a wonderful way to collect and you can set the boundaries where you like. The possibilities are endless.