Hi,
I’ve only recently started gliding again, after a 30 year break, and I have nowhere near your experience, but back then I did have some cross country experience and was lucky enough to fly a single seater in a regional gliding competition.
I was just trying to find some thermal cross section illustrations, to refresh my memory, I couldn’t find any but I did come across this
So this suggests that sink does indeed surround the column of lift in the thermal, but it is less strong than the rising column of air. My own experience even recently has me getting between 3 - 5 negative on the vario for the bits where i’m not in the thermal for a the full 360 because I’m not quite centred.
My expectation would be that the stronger the thermal, the stronger the sink surrounding it, although not as strong as the rising air in the thermal, but still at times, really quite strong.