My idea is to start an online/ mobile app platform that would allow younger people or people with smaller amounts of money they’re willing to risk in an investment to crowdsource properties in large groups and collectively buy properties outright.

Glad to have you in this platform.

I'd just recommend using as many listing platforms as you can.

The new focus for short-term rentals is building your own brand outside of the Airbnb platform anyway.

It's exciting to have owners, investors, residents, students, etc all join this great platform – in order to grow in whatever their area of interest is.

A handful allow a month or more...not day by day....some people say renting 30 days or more to one tenant can change the STR to someone with tenant rights, so some operators won't do that.I've also seen condo associations prohibit or restrict even LTR, so that does happen.You could be entirely right that state law supersedes HOA CCR and R&R, but is that a fight you want to fight and pay to fight and gamble on big lawyer fees?

In addition to the platforms you mentioned above, check out Wanderly and Furnished Finder as well for popular medium term rental sites.

Any thoughts on using investment platforms like Roofstock and crowdfunding platforms like Fundrise as an investment strategy?

It's all a gamble, barrier of entry aside. 

But yes I’m gonna be posting a lot on this platform for feedback this app is amazing