Hi Ben
Whether serves are hit above the waist or not is an ongoing debate at all levels of the game.
It’s interesting that at the recent Commonwealth games quite a few players had their serves called particularly when flicking.
Credit to you that you are concerned at an early stage and not, like quite a few players, braving it out until the evidence is overwhelming.
The serve is like any other stroke in that small nuances can creep in over time without us really noticing.
Because of infrequent use players can get a bit anxious, tighten up and subconsciously modify their technique with flick and particularly drive serves.
Paul calls the serve, quite rightly, the most important stroke in badminton so it pays to practice.
You’re almost right on the rules.
The flight of the shuttle shall be upwards from the servers racket.
The shaft of the racket shall be pointing downwards at the moment of impact.
What a lot of people don’t realise is that the WHOLE shuttle should be bellow the waist defined, as you said, by the lowest part of the bottom rib.
If only the base is bellow your waist when you strike the shuttle then the the serve is illegal.

Everybody develops their own service position and action and,as long as it’s legal and effective, that’s fine.
When I’m teaching the backhand serve I get my learners to place the shuttle onto the strings of their downward facing racket and then extend both arms making sure that the shuttle is bellow the waist. They then look at the target area then down at the shuttle until impact. By looking at the shuttle when delivering the serve they can check that their shuttle and racket arms are in the correct positions and don’t wander during the serve.

Hope this helps
Roger