I can relate to this - I have developed online (video) courses and having to distill knowledge down into the bare essentials was something I struggled with initially.

Armando’s answer is a good one - you should look closely at that.

I would add the following:
* You say you end up with long chapters when you try to create a course. That indicates that writing a book may not be so difficult for you. Your publisher/editor would work with you to cut out the fat, and so that part of the work will also be taken off you.
* If you do write a book it’s contents will be easy to convert into a number of training modules, if you still wish to do that after becoming a published author. The structuring of the book in partnership with your publisher would have clarified a number of questions about the specific problems you are helping people solve and the issues you can address.
* You say you’ve already got a huge trove of video material. So how come you sound as if you want to start from scratch again? Or is it that you don’t own the rights to the materials you’ve done in the past? Simply repurposing the existing videos (breaking them into smaller videos and perhaps adding some commentary to update the material or provide further context) sounds like a very quick way for you to get materials out with little effort while avoiding the temptation to ramble.
* The medium often determines the message format.
— Udemy and Skillshare are good if you don’t have your own traffic, but you can’t put the same format on both. Udemy works with lectures that are 5-10 minutes in length, while Skillshare tilts towards 2-5 minute videos. You sound more like a Udemy guy!
— If you have traffic already then I want to assume that you are already conversant with all the good stuff you can do with Youtube. If you want to have your own platform to which you can direct your traffic I’d recommend Thinkific. Teachable is good too though.

You have a chicken-and-egg (mutual baiting) problem, not a marketing problem per se. And in building your platform you will run into other generic platform problems after solving this one e.g. the ghost town problem.

The good news is that there are a number of well-defined solutions to these problems; I'm aware of about 8 solutions to the chicken-and-egg problem. I'll provide some quick thoughts based on the little I can randomly glean/guess from your question but I would need to know much more about your specific platform to make a well thought-out suggestion:

* You mention "user side". So this may be connecting users of some kind to merchants of some kind?? A common approach in such scenarios is the "marquee merchant" approach where you get one respected merchant (like an anchor tenant in retail shops) that can be the magnet to get sufficient users on board, and then you can use the growing user base to get more merchants. Obviously you have to be willing to make certain special concessions to the marquee merchant(s).

* You mention that you have traffic. Another common approach is to focus on the side that is easier to get. So the question is: where is most of your traffic coming from? Which kind of participant (the term "user" is vague - you should be talking about "producers" and "consumers" in a platform) is predominantly coming in?
-- If you are getting mainly consumer-side participants then maybe you can create supply-side proxies like Yelp did.
-- If you are getting mainly supply-side participants then maybe you can figure out standalone value for the producers, like Opentable did.

The more info you make available the easier it will be to help. Good luck anyway.