I agree with all of this.Totally different.
I didn't see enough of Johnston to rate. Lade > Primus >>> Brogan over the journey.
Primus was such a dominating ruck prior to the rule change, but Lade was such a clever and effective footballer and held his own in competing for AA honours against 2 of the greatest ruckmen of all time in Cox and Sandi. Lade had the body of the old fashioned lumbering ruckman combined with a brilliant football mind and terrific skills for his size.
Ryder is probably ahead of Brogan but behind the other two. If I had to pick 1 from our AFL era, it's probably Lade.
Johnston was an excellent ruckman for us (& fine captain too). On face value he would have been a back-up ruckman for Primus and Lade, perhaps on par with Ryder & ahead of Brogan for the position. He was a skinny dude, so unless he was able to bulk up he would probably have trouble against the more modern professional bigger-bodied guys. Tantalising to think what it would have been like to have him in our AFL era.