To consider your solution, first showstopper, it needs a server. We don't have a server, nor anyone who knows how to manage one. We'd need to ask IT, that will take months and they'll require a budget transfer, so we'd need to request it to management (which will need a business case to convince) and involve the finance guys. We can't just plug a RaspberryPi into the wall, not only that would get me fired, but also it wouldn't be able to connect to anything without the company's certificates for the proxy or whatever.
Second, we need people who can code in PHP (and their backups when they leave). Probably in practice we'd need IT to do that, so that's more months and budget required.
Obviously anything stored in the cloud is out of the question, just the authorizations and contracts to do that would take a year.
So in the end it ends up as a shared spreadsheet.