As a pianist for opera and musicals I've played a good bit of Sondheim.
I can see three possible fingerings:
- Yours. Fine, needs a mobile thumb with quite a bit of in and out movement between black and white keys.
- Keep 5+3 for the chords but use the second finger on the F#s in the melody. The hand doesn't change position so much.
- Play the chords with 4+2 and melody with thumb. Similar to 1, may suit some people better if they are ok with the hand stretching more.
- 5+4 on the chord, leaving the other three fingers free to play the melody. You can play the Gs either with the 3rd finger, or with the thumb. This gives a nice hand position, but playing the chords with 5+4 may get tiring.
Number 2 is my preferred fingering: not tiring, and gets rid of the busy in and out movements caused by playing all the melody notes with the thumb. Whatever fingering you use, the melody needs to be heard clearly, while the accompanying semiquavers pulsate softly.
By the way, the E in the right hand in the middle of bar 145 is missing a stem: it's part of the melody.
