Hi Tom
Whatever, 300ppi is wrong for an Epson printer, 360 would be better because the native resolution is 720ppi (fine detail set) or 360ppi without that set.

EDIT: remember, an image does not really have pixels per inch, just pixels. It's only when the image is printed at a particular size that ppi become real.

1. You are better off to let Qimage do all the re-sizing and interpolation, just once. That's what it does best.
If you can cope with the large file - no need to bother about any tagged ppi and size - use that file in Qimage and set the size in the Qimage way with inches. You can then leave the output resolution to either the "native" value as reported by Qimage, just above the preview, or to reduce processing time, use a lower value, set at the bottom right of the main screen. The lower values offered are whole fraction of the native resolution and with very large prints, may well be adequate.
2. I'm not familiar the Bonzai software but if you are wanting to reduce your file size, 72 pp seems quite low. 180 would be better and it is a whole fraction of 720-360.
3. Another approach would be to make your image pixel size such that the file is as large as you want to go and let Qimage do it's magic.
Terry.