The latest in my search to pinpoint exactly where one of my Great-Grandfather's "Pahiatua Seed Company" businesses was in Pahiatua has lead me to believe that my original guess was most likely close to correct.
It's been a process of digging through https://paperspast.natlib.govt.nz/, looking for old adverts and going through old photographs, maps, and books, and the cooperation of museum and archives staff.
Advertisements first appeared in the Pahiatua Herald on 8 September 1893, with the last being 17 December 1907.
The most specific address in the adverts was "Opposite the Bank of New Zealand, Main Street, Pahiatua" with their seed and trial garden at Hall's Road.
I've narrowed it down to somewhere between 104 and 110 Main Street, most likely 108 Main Street.
The property directly opposite the BNZ was vacant at the time, and 143 Main Street (on the corner of Main and Princess Street) was occupied by a store named Cross and Sturton. 120 Main Street (diagonally opposite the BNZ) was the Bank of New South Wales.
This 1892 map shows this section as a corn store, between lawyers' offices on either side.
I was in contact with the current owners of the Black Stump Cafe (106 Main Street), as their website states their original location (171 Main Street) was situated in a former seed and grain store. For a moment I had entertained the idea that the cafe was where Pahiatua Seed Company was, and as they've just put the cafe on the market, very, very briefly entertained the possibility of owning the location of Pahiatua Seed Company.
108 Main Street is part of the premises of Bisset Honda and Stihl Shop, and they have said there's an in-ground safe at 108, but don't know much about the history of the site.
Update 28 October 2021:
In late 2021, I was contacted by an organisation developing school resources for the New Zealand history curriculum, who had been reading the Wellington Chinese History wiki, and following my journey of discovery, especially the story of Chin Ting and the Pahiatua Seed Company.
They wanted to know if they could point students to the wiki, as part of some resource activities. Subsequently a team went to Pahiatua, and after meeting with people at the Pahiatua Museum, and looking at some of their resources, came to the conclusion that the location was 90 Main Street, Pahiatua.
My mistake, it seems, was to take the "Opposite the BNZ" too literally!