Integration ux – How to collaborate and define a new style guide for an existing business design?

As a naïve designer, I have a hard time defining a style guide. There is a style guide created with Adobe XD. As a team effort, we would like to improve and manage the style guide. A reasonable and productive solution is to use Frontify. Unfortunately, the company is running out of budget and we have to look for an alternative that can provide the same functionality as Frontify.

Anyone facing the same problem?