My personal choice for the past 24-years has been Photoshop. I started out with CS-2, upgraded to CS-3, then CS-5 - all of which I owned. A few months ago my old Win 7 computer died and I was forced to 'get with the times' using Window 11 which meant my old Adobe software would no longer work.
 
I went with Photoshop Cloud at $22 a month. Love it!
Adobe Illustrator, also available via cloud is used by many as well.
 
Now, if you are new to using these types of tools, then you may want to try out one of the latest versions of Photoshop Elements. You can do many things and own the software for under $100.
 
The reason why GCU won't accept Canva images is because the artist is using elements from Canva to create a design of which they will make a profit. So, if you don't use your own elements to create, or don't purchase CU licensed 3rd party graphics to create, then you need to find a design software which allows commercial use of the creations.

I switched from Adobe Illustrator to Affinity Designer 2 this year and have been very pleased. Every now and then Affinity has a big half-price sale on all three programs in the suite, Designer, Photo and Publisher. I use all three but mostly use Designer for card designs. It is very similar to Adobe but without the monthly subscription.