As you are paying with Oyster with 16-25 Railcard discount encoded instead of contactless, there is no such thing as post hoc extension fare like contactless, you will be considered as capping at z1-6 once you made the journey from/to West Drayton.
Using your hypothetical example, assuming you travel your whole itinerary in reverse order:
Camden Road to Stratford: £1.05
Stratford to London Bridge: £1.70
London Bridge to Belsize Park: £1.70
Hampstead Heath to West Drayton: £1.25
U3 bus: £1.65 (not capped at z1-2 because you went to z6)
423 bus: £0 (hopper fare)
Total: £7.35 below off-peak z1-6 railcard cap of £9.30
If you were holding adult Oyster or contactless:
Camden Road to Stratford: £1.60
Stratford to London Bridge: £2.60
London Bridge to Belsize Park: £2.60
Hampstead Heath to West Drayton: £1.90
U3 bus: £1.65 on Oyster (but would be partially refunded later) / £0.90 on Contactless (as you have now reached z1-2 cap with a z3-6 extension fare)
423 bus: £0 (hopper fare)
Total: £9.60 (£0.75 will be refunded on Oyster after two weeks or when £1.50 refund is reached)
