I'm a CRI (albeit with a CPL) with FCL945 and instruct freelance and in a couple of schools and syndicates.
Is there a lot of work? Not a huge amount, but enough to keep me entertained. Last year I did just under 50 hours instructing. However, at any time I've usually got a couple of students on the go, and it's worth doing - it's usually stuff that doesn't significantly affect the core FIs income which as you say are usually ab initio courses and trial flights. NPPL SSEA <--> M conversions, biennials, and checkouts are my bread and butter. I could do a lot of tailwheel conversions if I had an aeroplane suitable, but I don't.
So yes, the work's there, but only a limited amount. You are massively reliant, unsurprisingly, on the relationship with the school owners - they need to like and trust you, or you'll get no work at all.
G