For promotional purposes, the format does not matter much.
If you want the best bang for your buck when it comes to marketing your own stuff, short video formats like Instagram's Reels and Tiktok are currently the best way. And for that, it really doesn't matter if your comic itself is vertical or traditional because you're gonna be showing zoomed-in panels. Similar with other social medias, you're going to be mostly posting zoomed-in panels of your comic anyway.
For gaining Patreons, that's also completely unrelated to the format. There are tons of successful patreon artists who do only vertical or traditional formats. Patreons is more about finding readers who want to financially support you, and rewarding them with early reading, extra illustrations, and/or other rewards.
If you want to print, then it is best to draw your comic in traditional format and either upload it as is, or reformat it separately when uploading to vertical webcomic platforms. I currently have no plans to print, so my comics are drawn in vertical from the start.
When it comes to Patreon and printing your comic, you do have to consider your audience demographics. Older readers will have the disposable income to pay you while younger readers won't. But younger audiences are great at spreading word about your comic, as they're generally more excited to share things they like via memes and screenshots, which gets the attention of the older audience.
