Nextcloud server has 907 contributors, and 2672 Open issues and 37970 Closed. That is without the desktop client, the mobile apps (multiple), and everything around (viewers, editors, voice/video/IM app, and so on. It's packaged in 101 different ways, enough of them being all kinds of one command/click/etc. install, and you can even buy it directly managed and hosted and everything from multiple providers, including (but not limited) from NextCloud themselves. I think it's extremely hard to compete in any way, even if you have something much simpler.
Data Hoarder
We are digital librarians. Among us are represented the various reasons to keep data -- legal requirements, competitive requirements, uncertainty of permanence of cloud services, distaste for transmitting your data externally (e.g. government or corporate espionage), cultural and familial archivists, internet collapse preppers, and people who do it themselves so they're sure it's done right. Everyone has their reasons for curating the data they have decided to keep (either forever or For A Damn Long Time (tm) ). Along the way we have sought out like-minded individuals to exchange strategies, war stories, and cautionary tales of failures.
It let's you choose your storage directory and upload/download to and from it over the internet. That's it!
VPN + network share...?
Right now I'm thinking $9.99 for a LIFETIME purchase DRM FREE. Is that reasonable?
From the single sentence explaining what it does, absolutely not. I can choose a directory and share it within 10 minutes.
Alternatively I was wondering if maybe instead I should try to sell cloud storage itself?
You aren't the first person to come up with this idea, nor will you be the last.
a way to finance getting deeper into it
Better finance some lawyers first. What are you going to do as soon as you get a DMCA notice? Or if one of your tenants store something super illegal? Lmao
Those questions are why I'm hesitant about hosting and would prefer to just sell the software to host. Also, your profile says you have a huge setup. Running your own cloud is obviously something you can do, but most people can't. My app would be for them.