How to achieve greater code refactoring automation?

Since you already have Git, Codex CLI, Playwright, lint/smoke, and checkpoints, I would not try to make it fully autonomous first. The safer speed-up is to move one complete loop onto the staging box and make Codex own only that loop. A workable pattern is: create one issue class at a time, for example one PHP 8.3 fatal/error family, let Codex inspect files and propose a small patch, require it to run the exact lint/smoke commands, then force it to stop with a diff plus command transcript before commit. Keep ChatGPT for deciding the next batch and reviewing failures, not for copying every command output. The main guardrail I would add is a per-task contract: allowed paths, max files changed, required tests, and a rollback/checkpoint command. That should remove most paste-back work without giving a tool permission to rewrite the whole modernization in one run.