Re: So, for those of us who are a bit thick...
"This is it in summary, yes? "
Correct.
"The only real issue is going to be lifting a 150t spacecraft into orbit in the first place"
That's not exactly trivial but there are options.
" (and presumably telling a lot of people not to look at the sky when it starts up, since that would be like staring into a teeny weeny star)?"
People look at the Sun (for very short periods) fairly often and (as long as it is short) that does them no permanent harm.
"Please someone tell me the downsides."
Well the scale up is pretty substantial. The power array would be the 2nd biggest after the ISS (most PVs on sats are 1/10 the size at most). So far (IIRC) they've demonstrated single shots with Al rings without fusion and this mechanism has to deliver these rings at 1 a minute over 2 three day periods (which is a serious mechanical engineering problem in a space grade vacuum), coupled with dumping enough energy into the pellets (and actual pellet mfg on this scale is pretty substantial as well).
I guess the key question is how viable is sub-breakeven fusion. This is the critical bit that all those rings wrap round to give the thrust.
A comparison with the VASIMIR concept (to see what things it shares) might also be useful.