With the proposed changes to bundling languages with Txp from 4.7, propose landing on Diagnostics by default (perhaps a little prettier). With:

• "Welcome to Textpattern version x.y.z" at the top if all went well.
• List of "installation issues" (scraped from upgrade scripts) if anything went sideways.

Then pre-fight checks as now, which highlights the 'remove setup' thing, and any server-level issues that might need addressing.