If I read this right, setting off for the Golden Circle at 2, trying to get back to Sky Lagoon at 7
doesn't leave you much time.  I did the whole thing in a day, so if you gave up snorkeling and
started off in the morning, you could probably see everything in a day without too much stress
and get to the Lagoon by 7.
Taking 38 -> 427 -> 43 to the Lagoon would avoid having to approach Reykjavik and might save
you some time.   But there are always things that get in the way.   I hit a road under construction
that wasn't designated on the map, and you want to leave time to veer off the main road and see
things you didn't know were there, like a small church.
You also don't say what time of the year you're going.   Sunset will impact driving time a bit, as
well as crowds.  Thingvellir may not be that crowded, but it is large and takes a while to get
through/around.  Gullfoss is easily an hour or two.  Maybe an hour for Geysir.  If high season,
add time for crowds and traffic.