Every time I'v tried this approach it is a higher price due to the flight being one-way.
My best advice is to do the following.
After you find flights that fit your budget,
go to those actual airlines' websites.
Often you can find a lower price, or the same.
But the best reasons to book through the airline vs the search sites are many. You are allowed to prebook your seats, allowed to upgrade, can change your ticket more easily, and can deal directly w/airline if something goes awry. Yes, you are charged a fee.
When I have used booking sites to buy tickets, I do not either have those options or it's difficult at the last minute to make changes. Nor do I have someone to assist me if something goes awry.
I look at the price, the type of aircraft, the length of time for each leg, and my loyalty programs.
Depending on where you have flexibility then make a decision based on what fits your budget,need and style.
For me I'd pay the extra money because of the long trip hours for your particular travel, and fly the AB-330 both ways.
My choices would be in this order: 4-2-1-3.
I do not like the larger B-777, even though I could have my own t.v. and power port. I prefer less people on the plane.
I prefer the B-767 and AB-330.
When I travel with someone it's nice to have two seats alone vs sitting in a 3 seat section.
I bring my own toys to play with.
Whatever you do, Enjoy your travel and Happy Anniversary!