I had considered PVC but didn't find any that was 5/8" as you
suspected. *If I went with 3/4" PVC could I just caulk where it meets
the siding instead of using a flashing? *Also, I presume the
hardiboard would last a lot longer than the plywood siding did, right?

the other idea i'd had was to use a piece of pressure treated lumber,
although I don't think I can get that in anything narrower than
1" (e.g a 1x12).