> There are two so-called gospels of Thomas: the one called the Gospel of Thomas, which is a collection of sayings attributed to Jesus, and the Infancy Gospel of Thomas, which is what we're talking about here. The Infancy Gospel is dated from between AD 150 to AD 185.
I don't believe anyone would confuse these two gospels. Infancy gospels are always specified as such, and the Infancy Gospel of Thomas is never referred to as the Gospel of Thomas.
> Also, the Gospel of Thomas has a huge range of dates of composition,
All the gospels have a range of estimated dates of composition, though the Synoptic Gospels have consensus for a tighter range, the date of composition of John spans at least 20 years.
> partly because it is just sayings, most of which are found in the canonical gospels.
And this is key, especially considering the hypothetical Q Source, which it is believed that Matthew and Luke rely on apart from Mark. The Q Source, material in Matthew and Luke not found in Mark, is a sayings gospel, and could very well be a different version of the same Gospel of Thomas. If true, Thomas may indeed be the oldest gospel with a date of composition around 50AD and that was only passed down orally until written down later. Or it is possible it was written down early and no copies survive.
> AD 60 is the earliest date of composition,
As I already precisely specified....
> but some time in the mid-2nd century is more likely.
By what reasoning or evidence? If Matthew and Luke rely on Thomas, obviously Thomas would predate them. Some believe it was composed in the mid-3rd century, but if it is the Q Source, it would necessarily have a far earlier composition date. There are many theories and no overwhelming consensus. One theory hypothesizes that the Q Source and the Gospel of Thomas have a common source.[1] And it is curious that a sayings gospel would be hypothesized from analysis of where Mathew and Luke agree against Mark, and such a gospel, a complete copy of the Gospel of Thomas, was already discovered but not yet widely known.
[1] https://en.wikipedia.org/wiki/Common_Sayings_Source