>> I will be staying in Zürich so I will have to leave REALLY early that morning to travel through Luzern and on the Brünig Bahn to reach Meiringen by about 9:00 am.
Here are a list of the connections that will get you from Zürich to Meiringen in time to catch the Vierpässefahrt bus, together with some comments about each:
1. Zürich-Meiringen via Luzern and Golden Pass route
Dep Zürich 6:04, Arr Meiringen 8:16
Sunrise is between 7:18 and 7:29 depending on date. This will be between Luzern and Sachseln. You will miss some nice lake scenery, but should still have a good view of the Lungernersee (Lake Lungern) and good views from the Brünig Pass, but there may be deep shadows in the valley.
2. Zürich-Meirigen via Bern, Spiez and Interlaken Ost
Dep Zürich 6:32, Arrive Meiringen 9:07
Sunrise will be between Zürich and Bern, so you won't miss the best scenery, which will be along the Thunersee (Lake Thun) and the Brienzersee (Lake Brienz). You would not be travelling along the Golden Pass route though.
3. Zürich-Meiringen via Luzern and Golden Pass route
Dep Zürich 7:04, Arr Meiringen 9:16
Sunrise will be between Zürich and Luzern, so you won't miss the best scenery, which is between Luzern and Meiringen. However, you would arrive only 9 minutes before the bus leaves. If other people have started boarding the bus, you may not get your preferred seat. A reserved seat guarantees you a place on the bus, but not a specific seat.
If it were me, I would travel from Zürich via Bern, Spiez and Interlaken Ost, arriving in Meiringen at 9:07, and then take the Golden Pass route back to Luzern at the end of the day.
If you got the 18:42 from Meiringen to Luzern, you would get to Luzern just before sunset and should be able to see the views from the Brünig Pass and of the lakes between Meiringen and Luzern. You won't miss much in the dark between Luzern and Zürich.
Luzern is lovely after sunset, with all the reflections in water, so if you feel like taking a break there for dinner and a stroll across the bridges and through the old town, that would be worthwhile I think.
The Rathaus Brauerei restaurant has a lovely riverside location in the old town not far from the railway station, and I can recommend it from personal experience for traditional Swiss food and ambiance. If the weather is nice, there is outdoor seating under the colonnade or on the promenade right at the river's edge.
One night when I was at the Rathaus Brauerei, the weather was warm and I had hoped for a seat at the river's edge, but they were all taken, so I sat under the colonnade. Before long, a sudden downpour had all the riverside diners running for shelter, so I was glad of my table under the colonnade after all!
I have attached some photos I took that night.