If you're paying a subscription for anything, it's unlikely to be something you'd want to replace for an internal tool. Yeah, you can easily vibe code the basic functionality of a Slack/Jira/Confluence/Ashby/Salesforce replacement in a workday now, but 1) there's no chance you capture all functionality needed by your business on the first pass, you're going to be getting inundated with feature requests as soon as you roll it out, and 2) the ongoing maintenance cost is going to be much higher than expected. Not just maintaining a server and database, which is cheap, but training people how to use your tool instead of the industry standard one. There's a reason Atlassian and Salesforce stocks have recovered after the theorized "SaaSocalypse" which never materialized, it's just not an overall cost savings to build this stuff yourself.

Where internal tools shine is internal operational workflows that are being done manually. You're not replacing Slack or Salesforce, you're replacing the current process where a developer needs to SSH into the server to run a SQL query, or where the Operations team fills out a Google doc which the Finance team pastes into an Excel spreadsheet which runs a bunch of VBA calculations.