With Ignition 7.9 and the early versions of Ignition 8, the certificate renewal process could be accomplished outside of the Ignition Gateway configuration screens. Files could simply be copied to the correct folders/directories and would be effective after a restart. Starting with about Ignition 8.0.15, this process no longer worked and the gateway config web server screens needed to be used to generate and renew certificates. We would like to be able to manually install files. This will facilitate scripting (external to Ignition) that can automatically request, renew and install certificates prior to expiration. Perhaps, all that is needed is an accurate list of where all updated cert related files need to be placed.
The Web Server config page was added back in 8.0.3, but it is still possible to automate the cert renewal / installation process like you mention. In fact, whatever process you used in 7.9 should work in 8.
If you haven’t already, check out this guide: Let's Encrypt Guide for Ignition | Inductive Automation