You should use timbor or boracare.

Not at the prices I am seeing for that stuff. But I can see it's just
a borax product. Why not just use boric acid?

Well, boric acid will work, mostly, but "just a borax product"
is kind of like calling mortar "just a calcium product".

Aside from being a different chemical, boracare has some
sort of penetrant (ethelyne glycol?) to get it deeper into
the wood.

If you're buying antique carriages at auction, doesn't it
seem strange to be quibbling about $40 for preservatives?