Their first four albums are all pretty damn consistent to me. A couple merely-good songs here and there, but even then they hardly ever rewrite the same song over. I think the lyrics are the one weak aspect of their early material; they don't ruin anything for me, but I'd much rather just listen to the tone/conviction of his voice than actually read along to the lyrics. In a way I can see why he'd be considered inseparable, but trad doom is arguably also the sub-genre where vocals are the most important anyways. I.e. it's personal music so you tend to latch on closer to what you're used to, but I think in an alternative universe where Psalm 9 had a different singer, it would still be a classic album and whatever vocalist that stepped in would hopefully put his own mark on it. But without Wartell and Franklin, Trouble is nothing.
EDIT: fwiw I think Kyle Thomas was a great fit for the new Trouble album, and for the handful of strong songs on it, I could totally still tell I was listening to Trouble. There's also a bit of average stonery buttrocky filler, but that also applies to Simple Mind Condition.
This is reminding me that I've never actually listened to The Skull, the band, though. I should get on that and see how it works.