When you make a comic for the first time, you make a lot of mistakes. You learn a lot. You learn what are the best ways to approach a story, what is the most efficient style of workflow, what does not work, etc etc etc.
A short comic to start with would be very helpful in terms of learning how to create a story from start to finish, and in the process develop your "comic style" as I like to call it. (AKA the more efficient version of your usual style. Coming from the background of an illustrator, you end up needing to learn how to simplify the style to work for comics and yet still produce a pleasing result.)
That being said, I don't think it should be a hard rule that you should NOT make long comics as your first one and you MUST go with a short story. I highly disagree with this advice, and it ultimately misses the point.
The reason why people recommend to not start with you "magnum opus" or start a "short story first" is for the reasons I stated above: you learn a lot in the first few chapters of creation, and you want to make the mistakes in a disposable way before moving onto the "real thing". But here's the thing people don't realize when they offhandedly make this advice: Comics have a very high barrier to entry from its sheer difficulty - the multiple skillsets required, and the endurance. I have seen many, many talented artists and professional illustrators attempt and give up on comic making because it's a long term commitment for an undefined payoff. Every illustrator is an artist, but not every artist is a comic artist.
The most important thing is NOT that you have to make mistakes and learn in a "disposable" way through the form of a short story. The most important thing is to have the passion and love for your story that will motivate you to work on the story for the long term. Nothing kills motivation quicker than working on a story you are half hearted about on such a difficult comic venture. I can tell you this, many will realise how hard it is, think they must not be cut out for it because they can't even complete the short story to move onto their passion project, and they give up entirely.
If your passion project is a short story, all the power to you. If your passion project is a long epic, then start that journey and learn, make mistakes, and keep moving on. The more you learn, the better. You can always reboot the story years down the line with your already developed fanbase when you have gained enough experience. You do not need to go through a half hearted short story to learn these things. And who knows, maybe you have a better comic idea and you decide to pursue that with all the experiences you have with the previous epic that you were passionate about.
I quite disagree when I hear people try to advise not to start with your "magnum opus" and to start with a "short story" because it makes the beginning passionate comic creators feel like they are doing something wrong by pursuing an epic they hold dear to their heart. No matter how many short stories you make, you will keep making mistakes and keep learning. So you might as well just have fun along the way.
Of course, everyone is different. Some like making short stories, some like making long ones. Ultimately, the most important rule is to love your story because that's going to motivate you to continue this grueling neverending comic grind process.