Overall, I think this idea is a great one, though the stack still needs some development/improvement. To second hatware's challenge, if you can come up with a (mostly) turn-key solution, one that would be as cheap & time-thrifty as hosting on cloud service starter tiers, you could own this segment and lure the new startup demo away from the cloud providers.

Many people seem to forget that there are no free lunches... anyone who has ever wrestled with the learning curve of a necessarily proprietary no-code solution knows that you don't usually save time or money by outsourcing your platform, and that by outsourcing, you can easily become indentured to a restrictive black-box environment you don't control.

If you're not dealing with critical security updates to your Mattermost install, you will be dealing with something equally time-consuming and fundamental, like managing CloudFormation templates or trying to get divs to nest correctly in a WebFlow one-pager.

If your startup doesn't have the in-house knowledge/intelligence to be able to run a stack like the OP's, you might ask yourself how you plan to create a successful software company without any in-house software competency.