> This plus the fact that Dependabot is not able to update Docker Compose files at this time.

https://github.com/dependabot/dependabot-core/issues/390 upvote and/or implement it as a fork and run it in an Action.

> The other problem that I have is there is no easy way (probably for good reason due to potential abuse) to manually trigger Dependabot. So Dependabot might run once a day or not do so for quite a while.

There is, it's just hidden in https://github.com/:org/:repo/network/updates > click "last updated ..." and then click "Check for Updates" and it'll start a manual run.