for beams like this, I often use a simpler solution than creating an "all purpose" family.
As you will not be having to create this kind of beam in majority of cases, think about using the "create insitu" component.

First place a section perpendicular to where you want to place your beam so that in theory, you would see the section you have shown above if it existed in the project. Make sure you note the section reference number you just made so you can use it to draw your profile later.

Go back to the plan view based on the reference level you wish to create your new beam profile.

In the structure tab, "component" then "insitu" make sure you classify it as either framing, or foundation (depending on its use, but as its a ground beam I am assuming, make it a foundation) This is important for your schedules later.


Then you can either make it a sweep, or an extrusion, but in this case as it follows the angle of the slab you are likely to only need to use the extrusion.
Go to your section view
Draw your profile (it can be as complicated as you like, but in this case its a beam with a chamfer on the top)
Set the material to be the same as your slab
Finish the extrusion

Job done.