RT is what it is, you could add some styles to CSS to change things around a bit, or even write your own theme, but I’m not sure if anyone has ever done that.
RT 5.x (or 4.x with the REST extension) does have a good API so you could call that directly from something like PowerBI, or even dive straight into the database. In the latter case, be warned, it is gnarly in places.
Do you have experience using rest api and PowerBI?
Not in recent years. I last looked when we had the REST plugin installed on v4.x, which is now native in v5.x. PowerBI will have improved a lot since then too.
I will be looking more closely once we’ve completed our RT upgrade to 5.0.7