Update...

No significant blow-by and the exhaust gas recirculation was not blocked. So the donk came out. I fitted a new rear main and T seals and now it is all back together and leak free once more. Although the underlying cause was not immediately obvious...

A few observations:

The seal itself was bone dry. I used a magnifying glass and torch up under the bottom of the rubber lip and it was completely dry. The crank was dry all the way around. BTW - this seal was installed with a very light smear of grease, so I will be doing that again.

Whilst one of the T seals appeared wet, there was no apparent leak path to get past the seal housing.

A small amount of oil dripped down the bottom of the housing as I removed it. Some of this oil must have been sitting just behind the crank, yet it's plausible that I had a sealant failure (or I didn't apply enough sealant) at the bottom of the housing as well.

There was oil on the outer face of the crank (flywheel mating surface). Again, at least some of this oil could have dripped on the face as I removed the crank bolts, or it might suggest that oil was leaking through these threads. The outer surface of the flywheel (clutch mating surface) was dry.

Any thoughts as to the cause? Considering the amount of oil leaving the engine, I doubt leaking past the flywheel bolts was the main cause or a contributing factor at all. So I suspect it was the sealant. I had used Permatex ultra grey. But it's weird that it held for 20,000km before letting go.