You are correct that there is no such thing as a standard circuit - as (mostly) the circuit is determined by airspace constraints (vertically and laterally), by noise abatement procedures and by local procedures.
Whilst prepping a student for first solo we fly a standard LH or RH circuit with reference to clock positions (8,4,2 and 10) and back up that with ground tracks. Much of this is confirmed (at home base) by reference to ground features to ensure noise abatement/airspace etc.
The difficulty in GA comes when you start to 'land-away'. What works at your home airfield may not fit in with the procedures at your destination. The generics need to be adjusted to fit what ground tracks and features you can gleen from Pooleys/AFE/AIP information and/or a PPR briefing.
When turning final its a variable feast again. We aim to start the turn at about 500' AGL rolling out about 350' but where the turn begins is obviously a factor of wind with reference to the position of the runway in the side window (vertically as well as laterally). In our SOP's it's a MAX 25degree AOB turn onto final, but thats a minor point. And a 3degree glideslope.... mostly a myth in GA.
I'm sure that you'll appreciate all of that anyhow, but there are far more variables at GA airfields than there are at military - and far more 'standards' being applied. There is, unfortunately, no civilian CFS!
HTH
DD