Some ideas:
- Whatever you want to do: First take a backup from the site. Also check whether you can actually completely restore the site from the backup.
- Install a staging system that mimics the production system as close as practically possible.
- Install a vanilla WordPress on that staging system without Bedrock (composer).
- Import the backup into the now “vanilla” WordPress site. This is also a nice way to check whether the backup can actually be used for restoring the site. With importing backup I mean the WordPress site database and uploads/ folder - which is basically the WordPress application state.
- Install the plugins manually or using a plugin that helps with installing the plugins.
- Concerning the theme installation: When it is a Sage-based theme you may want to package it as a ZIP file and install it the classic way. These build scripts may be useful.