Freenom for Subdomains

I have recently pondered creating a platform where I register a core set of premium domain names that are sufficiently generic and then wrap them in an interface where users can register free subdomains off that core set of domains I registered. They would be virtually isolated and could manage their zone file records on the domain. It would basically be like what Let's Encrypt did for the SSL certificate but for domain names.

Freenom offers this today at the TLD level but they are known for taking domains away from people after they get sufficient traffic and I would not be interested in doing that.

I would offer a free plan where you could register a certain number of domains for free and then you can upgrade to a paid plan (maybe $5/month?) to register a larger batch and ability to add more zone records.

I'm just wondering how feasible this idea is? Looking for feedback.

The main concerns I have with this idea is fighting spam and abuse as well as TLD and registrar terms of service issues around any issues with this type of setup. So far I contacted ICANN and they said abuse would primarily rest with the registrar and as long as I respond to abuse in a timely manner it should be fine. I was thinking of using Firebase phone auth to make sure users don't easily create duplicate accounts. I was hoping this would be more automated of a setup and could be largely left alone once launched other than upgrades but I feel like a good portion of my time will need to be used to handle abuse and spam.

Thoughts?