if you have not already, read the online manual (several times).
Home Lock and RTH are two very different features. First a lesser note: RTH is when you lose your connection to the Mavic and Smart RTH is when you initiate the RTH even though you have a connection.
When RTH is started, the Mavic rises to your present altitude and flies back to the home point. It does this automatically. It will then hover and eventually land.
Home Lock is a flight mode that changes the direction the Mavic flies when using the sticks. Instead of the sticks being oriented to the Mavic itself, they are oriented to your direction when Home Lock is set.
What you are considering is that pulling the stick toward you means that the Maciv will fly toward the home point regardless of the direction it's facing. There are several differences. First, you need a connection to the Mavic using Home Lock. You don't on RTH. You then need to hold the stick toward you using Home Lock. The Mavic will not automatically rise up under Home Lock as it does under RTH. Lastly, you may be flying the Mavic to the Home Point backwards or sideways using Home Lock so the collision avoidance won't work. Under RTH the Mavic will fly back to the home point in a forward direction so collision avoidance will work.
These are two different modes. Again, if you have not read the online manual, I'd recommend reading it _several_ times. You mention above that you don't know if the RTH altitude can be changed. You need to set it for your flying area _before_ you fly! You _need_ to know that this can be set.
IMHO, RTH should only be used when you lose the connection to the Mavic. Otherwise I'd recommend that you manually fly it back to the home point. Some people don't share this view. It's up to you.
