You have the pixels > inches correct. 300 ppi for printing resolution, Gimp works in pixels so inches x ppi = canvas size.
If you plan on lots of images then it is really one template per size. 8x6 is not the same aspect ratio as 10x8
As racer-x explained, lots of layers. Text makes small layers and could go in a layer group.
A simple 8x6 template attached to play with.
I can just get demo into a 1 minute animation for imgur
https://i.imgur.com/ZIsjynv.mp4That is a copy - paste - resize in place - anchor floating layer. Add some text.