For the second point, you might want to implement some kind of browser warning similar to what Ngrok does.
Back in the day you could have stood up something like this and worried about abuse later. Unfortunately, now, a decent proportion early users of services like this do tend to be those looking to misuse it.
This is where an exfil (exfiltration) route is needed. You could just send the data to a server you own, but you have to make sure that there are fallbacks once that one gets taken down. You also need to ensure that your exfiltration won't be noticed by a firewall and blocked.
Hosting a server locally, easily, on the infected PC, that can expose data under a specific address is (to my understanding) the holy grail of exfiltration; you just connect to it and it gives you the data, instead of having to worry much about hosting your own infrastructure.
A permanent SSH connection is not exactly discreet, though...
Though the public address is going to be random here so how will the hacker figure out which tunnl.gg subdomain to gobble up?
However, if "No signup required for random subdomains" implies that stable subdomains can be obtained with a signup, then the bad guys are just going to sign up.