What is the minimum current for my load to see TTL?
The datasheet says, (2nd bullet point) that this device can drive up to 10 LS TTL inputs. The number of inputs you can drive will depend upon family, i.e. straight TTL, LS TTL, HCT, etc.

My loads' input impedance is 8.2 MΩ. So, naïvely, I would say that the minimum current for TTL to be &quot;seen&quot; is I = V/R = 5 V / 8.2 MΩ = 0.6 µA. Is this reasoning too simple?

Yes, your reasoning is too simple. A (standard) TTL input does not act like a simple resistance. It actually supplies current, when the input is low. The circuit driving it is expected to be able to sink that current when it is low.