It's not going to be 'overkill' to have a summit followed by a course launch on the same topic. In fact, that's exactly what you should do.
Since the point of the virtual summit is to generate leads for the course launch, you should create a summit on the same topic as the course so that you know for sure you are attracting and building an audience that is interested in that exact topic.
If you are going to launch a course about self-publishing, host a summit about self-publishing. Don't launch a summit about social media. You might think that the audience is the same, but even if there is some overlap, there is no guarantee that people watching a summit about social media care about self-publishing at all.
#2: Sell your course as an upsell in your virtual summit funnel.
There is no better time to sell a lead something than right when they're going through your funnel.
If you're selling a summit All-Access Pass as your front-end offer in your virtual summit funnel, which is typical, then those buyers have literally just 'told' you that they are directly interested in your summit topic.
They are 'hot' leads who are ready to buy, so if you give them a quality offer right then and there, many will take it.
That is why you should offer your course to them on an upsell page after your All-Access Pass order form. Some All-Access Pass buyers will buy your course right in that moment.
This doesn't in any way prevent you from selling your course to the rest of your summit registrants after the summit is over. In fact, you should, and you'll have to since only the All-Access Pass buyers would've seen this upsell offer.
That means the large majority of your total summit registrants (those who didn't buy the All-Access Pass) haven't seen your course at all still.
#3: Use a separate activity to kick-off the official course launch immediately once the summit ends.
During the summit, your primary call-to-action will be for summit attendees to upgrade their free ticket to the All-Access Pass.
Many of these summit registrants are new leads who are either cold or warm, so the All-Access Pass offer is a sensible way to convert them to customers. It's low in price and low in risk for the buyers.
Once your virtual summit is over, your leads are feeling motivated to take action after watching all the incredible speakers. You want to be the person to provide them that pathway to success (which is your online course).
Immediately after your summit is over, your course launch will begin, but your summit registrants don't know that yet. You need to transition out of your virtual summit and into your course launch by hosting an activity that makes it clear you are pitching a new offer to them.
A live webinar is a really great launch activity that works well.
The key with this transition to the launch is to address the summit registrants directly with copy that speaks to the experience they just had and what they want to accomplish next:
"The 30 incredible speakers on the summit gave you the insights into what's possible if you publish your own book. Now, I'd like to help you turn your dream of becoming an author into reality through my 'write your own book' online program!"
#4: Create an irresistible, limited-time course offer for your virtual summit registrants.
Allow the activity you choose in Step #3 above, like a live webinar, to mark the official start of your course launch's "open cart" period.
Like any live launch, you should have a specified period during which your leads can purchase your offer. Once the cart closes, that offer disappears.
The offer you'll be making to your leads during this launch is your online course, but you should be sure to package it as an 'offer', not just as a singular course.
In the offer, include items which a summit registrant might need or want to accomplish their goal. Including bonuses specific to this summit audience is recommended.