I feel you, and I have found this as one of the most impactful technical complications of the “bundling plugins with core” as well. We had a few forked plugins and it was very complicated to get them to work without removing the bundled plugin.
I don’t think Jay is suggesting that. A plugin can also override very specific parts of another plugin.
The best approach would be to convince the team that your code is worth being merged into the official plugin. That will work if your modification is generic or flexible enough. I see you already have made a fork and your changes/additions are pretty clean. Maybe the hardcoded “Flagged” string could be in a translation file and if you make :flagged default to false then you don’t have to modify the original event handler with an extra parameter but apart from that, it looks worthy. If I were you I would bring it up to date, open a PR and discuss this in the plugin topic.
If that route fails, you could simply build a plugin that overrides those three functions that you changed, and adds the on(:reviewable_created) handler.