As far as marketing it I believe that's largely just connecting with the right people (those with connections). Or sales specialist who would approach estate agents in the right way.
If this coding experience has strengthened you in software development, and you don't plan on quitting software development, then I wouldn't consider it a waste. It is perfectly normal and healthy to throw away code.
The downside(s) depends on the open source license you choose to use. If you use the MIT license you are authorizing commercial for-profit reuse of your code. There are many open source licenses to choose from so you can choose how much freedom you'll be permitting with them. Of course the downsides could be considered upsides.
As far as how much work; it doesn't take much at all. You can simply use
git to add all the project's file to a code repository on Github (or other git repositories). You'll need to modify a
README.md file to give people an idea about the code, project, and you vision for it.
It's a good idea to have a "first open source project". I can't speak as to the future of interest in it as I have my own narrow developer experience. But being active in open source will always be helpful to both you and the community.
One thing you should consider if there are any great "tools" you've developed that exist within the site. And if you can take that and separate it out into reusable code for other developers on their projects. These kinds of open source projects get much more traction: developers helping developers"
Blog about it, your vision, what you're looking for, who might be interested and why. Do many blogs on it. Focus individually on realtors and developers in blogs post (only one per post). Seek out other blogs or developers posts online who share you vision and link back to your blog posts or open source Github page. And if you'd like you can list that you're looking for committed dev help from a site like https://www.codetriage.com/