Ugh. I hate to be redundant, but the lack of threaded conversations here is really challenging. I would think that a comment parent ID would be trivial, but not having any idea what your architecture looks like, I'm sure it's probably more complicated than I think.
The interest based ads model has worked reasonably well for Reddit, but it wasn't enough to fully solve their revenue issues. Reddit Gold and Cake...Frosting?..seem like a reasonable premium model, depending on how efficient your stack is. If your interface starts lagging hard (and even the Plus had issues with this), your burn rate is going to increase faster than planned just on infrastructure scaling.
I don't think "Opt out of ads" is sufficient on its own for Premium, though. My uBlock Origins is doing an adequate job of that, so what incentive do I have to pay? Brand loyalty factors into the equation (and I have been a Reddit Gold user since it began), but it was a ten year cycle for Reddit to build up to the point of making any serious money off of Gold. Perhaps a more evolved Freemium model?
BTW, I'm not just a gadfly; I'm really happy to be here, and glad I saw Robert's post on the Plus. I've experimented with every new platform that's come along, but they were all deeply flawed, and certainly none of them have the pedigree you bring to the table. I sincerely hope this platform takes off.