If you type just exiftool at the command line, Phil has provided lots of examples.
exiftool '-filename<%f_${model;}.%e' dir
Rename all files in "dir" by adding the camera model name to the
file name. The semicolon after the tag name inside the braces
causes characters which are invalid in Windows file names to be
deleted from the tag value (see the -p option documentation for an
explanation).
Means that the new filename should start with the original filename, then added the title (iptc / xmp:title) and then the extension.
Something you are doing but have missed out of that spec. is "then add a space". Do you want the space if you have no title?