A client says they cannot afford a Full sandbox. How does that change your design?

It changes what I can promise rather than whether I can deliver. Without a Full sandbox I lose production-scale performance testing and the most faithful rehearsal environment, so I say that explicitly and then mitigate.

The substitute is a Partial Copy sandbox with a carefully designed sandbox template plus a scripted seeding process for anything the template cannot supply. That gives representative data and repeatability after every refresh, which covers integration testing and most of UAT.

For the gaps I would do three things. First, run data-volume testing against synthetically generated data at the scale that actually matters — usually one or two hot objects rather than the whole model. Second, treat the deployment rehearsal as a rehearsal of the runbook rather than of production scale, so at least the manual steps and their ordering are proven. Third, plan a longer heightened-monitoring period after go-live and agree in advance what the recovery route is for the risks I could not test.

I would also put the cost of the residual risk in writing. If the client accepts it, that is a legitimate commercial decision; what is not legitimate is an architect who quietly drops performance testing and lets everyone assume it happened.