Thanks to both of you for these suggestions, and sorry for the delay in my response. Got swept up in work and forgot to check back into this.
It looks like HF to GSP is 8km return, which should leave plenty of time for doing the 12km to TC at a leisurely pace later in the day. The fact that you can head straight down to Paint Pots from Tumbling makes this alternative look pretty promising:
- Day 1 PM: Paint Pots to Helmet Falls
- Day 2 AM: Helmet Falls to Goodsir Pass and back
- Day 2 PM: Helmet Falls to Tumbling Creek
- Day 3 AM: Tumbling Pass to TC camp and back, then down to PP car park via TC.
Does this seem more or less like what you are suggesting?