New water heater: 1/2" supply line

[Ravingmick]

Sorry if this has been asked elsewhere, my search didn't turn up any results.
My gas hot water heater sprung a leak so I have to replace it. I'm going from 40 to 50 gallons. Looking to get an A.O. Smith.
The main line into the house is 3/4", which splits via a 3/4" x 3/4" x 1/2" reducing tee, which goes into the hot water tank. The 1/2" line is around 10 feet long, then 4 feet down. It's connected to the tank via a 1/2" in x 3/4" in. copper reducing female adapter. While adding the new water heater I want to add an expansion tank.
Looking for people's thoughts on installing this. I could:
1. Remove the reducing tee and run all 3/4" copper to the new heater, then run a section of 3/4" out of the tank which reduces to 1/2" a few feet out.
2. Use all 1/2" and have a reducing female adapter on both the hot water heater and the expansion tank (has 3/4" connection)
3. Use a 3/4" x 1/2" braided line with a push shark bite connection and leave everything else alone...though not sure where the expansion tank would go/how it would connect.
4. Something else?

I talked with a plumber who said option 2 would be fine. I'm inclined to do 1, but it seems like a little more work/money. I'm happy to do what's best, but not sure what that is.

 

[GReynolds929]

Option 1 is preferred, option 2 will work.

 

[Twowaxhack]

Change it all out to 3/4 pipe. Install expansion tank on the cold side.

 

[Ravingmick]

Thank you both for your advice. I was a little nervous (probably unnecessarily so) about cutting the tee out and running all 3/4". I ended up going with option 2 and ran a 1/2" copper instead. Everything is up and running with no issues. I'm sure down the road I'll be switching out the main line completely with 3/4". Right now it comes into the house that way and pretty quickly hits a reducer.