I think I can add a bit to this, as someone who has worked on leveraging physical spaces to support ecosystems. Obvious caveats are there - I am doing this 8000 km away in India; the communities I engage may differ in focus / maturity / spending potential and purpose; and I leverage such spaces to create core revenue drivers (not just as space, but building on space), which may differ from your purpose. But there are some constants I do think stand true regardless of where you are or the purpose you take. Mini soapbox time, this is a bit long.
One thing I would advocate is how you approach this: a space-first or community-first model.
The difference between the two is quite stark. As a co-working and events space you can tend to define everything you do within the confines of the space you operate. While it allows you to focus on one clear are it can restrict you in becoming a provider of a one-dimensional value offering - space, space, and more space. You focus as much as possible in recovering space costs that might be great for the short-term, but can affect you in the long-term because the building gets typecast as that one-dimensional offering of being a space provider. I view this as a trap (which I have fallen in previously) only because I don't see space as a be-all-and-end-all of things and I don't think this realises the full potential if what you can do.
It's a great approach to club the space with a cafe, yoga studio, etc. as it increases the attraction factor and utilization of the space to multiple people, but for you as the operators of The Reef the opportunity is there for you to do more, and to better spread the risk of how you leverage space to earn more.
What you are catering to are communities. Take a community-first approach and you'll be working on how do you make the space integral but not exclusive for supporting different communities you can align with. You get more play and opportunity in how you work with them and that creates multiple revenue opportunities as you can start leveraging multiple areas of value to the communities you engage with. The most important thing is that you do not define sustainability with utilization of the space only, it can be programs, services, collaborations, leveraged on space, with The Reef being an added physical touchpoint that represents these activities - for a meeting, for events, for the living aspect, and so on. It also does not restrict you from just being a space open and accessible to all.
I think with this you will also see greater collaborations parallels between The Reef and Edgeryders. An easy way to breakdown the communities you can engage with lies with those areas of positive externalities in Brussels you identify, and which actors need to be supported / seeded / collaborated with for The Reef to create those externalities. And of course, the existing Edgeryders network.
I've got one other point on the partners approach, specific to the investor. I know next to nothing about the investment climate and profile of people in Brussels, and while I have an idea of how Climate-KIC operates, I'm not sure which aspect of Climate-KIC you see as a key metaphor. If you need external financing / support for the space, I'd explore the option of getting a group of private individuals / angels to support you based on the vision of The Reef. There is the issue of having a stakeholder above you, that you have some fiscal responsibility towards, but with a community and space angle you may find a more philantrophic set less focused on returns and more on supporting a purpose.
Last point on seeing the sheet. If you are looking to open it up as a co-working and event space you will have a few more line items for expenses: on the internet (helps to account for it separately as a line item), repairs and maintenance, food and stationery office consumables, etc. to account for. Again, no idea about the economics there, but they usually are small individually, but can definitely add up.
I'm quite looking forward to see how you grow this, will be keeping a keen eye on it