I know this is an old post, but I saw that nobody had answered. Here's my two cents:
Allow paid services and take a cut (but be prepared for the IAP and app store fee debacle). I know this is sensitive both in your use case and due to recent events in tech, but the flip side is that:
If you're not providing a value, why would people pay you anyway (applying to you and paid providers in your app)? Make sure the value is tangible to justify this.
Even time is money. Just because you can do something for free, that doesn't mean you necessarily should.
Free services are often a path to paid work. By enabling the latter, you're both adding and gaining value, while not removing yourself from the paid part of the equation.
Use ads minimally and unobtrusively (and with tracking kept to the minimum possible) for free services, possibly offering a cheap ad-free upgrade.
Even if you interest investors, they'll need to see a monetization strategy. They're not just going to give you money out of the goodness of their hearts.
If you do expect people to give money out of the goodness of their hearts, your only options are crowdfunding or asking for donations. Be aware that users may find either of those things more annoying than ads or paid services.
Whichever path you choose, you need to be prepared to promote.
I know this is an old post, but I saw that nobody had answered. Here's my two cents:
Thanks a lot for your words! I will take them in consideration