Upload & Sell: Off
Not everyone has the same definition of serious wildlife photography so you may want to start with your definition.
As several folks have said the 100-400 a great lens to start with; it is inexpensive compared to a lot of other options, light in weight, and capable of great IQ.
A lot of folks think the 300/2.8 is focal length limited even with a TC; but some like it better than a 500/f4 because they can manhandle the 300 and the 500 is too big, bulky, and heavy to lug around.
Which means you need to seriously look at your physical capabilities, are you able to swing a 500, or even a 300, around fast enough to keep a flying bird or running animal in the FOV.
I have the 400/5.6, 100-400, Sigma 120-300/2.8 (a great fast lens for some sporting venues with zoom the super great Canon 300 lacks), the 500/f4, and a Sigma 300-800. It is a close call as to which one I use the most between the 100-400 and the 500/f4. For IQ the 500 has an edge and as a rule I can manhandle it over a long day; but when I am in my kayak or hiking in the boonies I would take the 100-400.
A lot of conventional advice is to buy lifetime keeper lens for your bodies; and upgrade your bodies when necessary. My advice is that the 100-400 is a lifetime keeper lens that you could put on your 5d and use for long enough to make a reasoned decision about which direction you want to go in defining serious wildlife photography.