I wouldn't pay too much attention to a solitary 8-fer or something. Venky Prasad or someone random probably has one, when top bowlers don't.
The rate of WPMs - is that down to him playing those tours when others were out of it?
Yeah, wiki said he'd taken 5 fivefers but Cricinfo is showing it was only 3, so comparable enough to Garner rather than easily better like I said.
However, his WPM was 4.6, Garner's 4.4, Robert's 4.2 and Holdings 4.1
Usually WPM is sketchy for rating bowlers (Hadlee and Murali had no other ATGs to share with, McGrath and Warne had to share with each other) but considering these bowlers all played together mostly, it's somewhat useful. Maybe Garner and Roberts were softening batsmen up and Croft was picking up the scraps? Or just being used more often by Lloyd. But that all feels like reaching to try and take the success off him to fit a narrative
But to your other point, it looks like no, Croft mainly always bowled with at least 2 of Roberts/Garner/Holding, often all 3(with Marshall occasionally featuring)
Anyway, for whatever reason, Croft was the most frequent wicket taker of the quartet while he was in the team. Decent feather in the cap. Makes me think he must have been pretty close to their level
Edit - one explanation for the WPM I can think of, at least for Garner and Holding, is that when Marshall was established they had to share with the GOAT rather than just Roberts for Croft
However after '83 it was only a trio rather than quartet as Roberts and Croft were done, so maybe that's a moot point