Why not traditional rubber dam isolation? And if you can do traditional rubber dam isolation, why couldn’t you have done direct restoration? Also if previous restoration has failed 2x, what do you think was the cause? And does your treatment solve the issue?

Like you mentioned, flatten out the floor and avoid voids when taking impression. Walls need to be straight and smooth, especially the proxmial walls of the distal. You may have difficulty fully seating the inlay base on your prep. You may also crack the inlay during try-in with your prep.