The info out there suggest the Amiga version stopped at 5.0.
What info?
There is Lightwave 5.2 somewhere in the net, and the last Amiga version ever released was 5.7 according to Newtek, which was an update to v5.5.
It is a pitty that lightwave versions from 5.3 to 5.7 seem to have been lost in obscurity.