Nice run. What does your account churn look like, how long does a mass DM account usually last before it gets banned, and are you warming new ones constantly to replace them? That's usually the part that decides if the whole thing scales or not.
It's a mixture. Since I don't really expect for accounts to live long (their lifespan is literally 5-15 actions) I mostly register them via API actually. With phones it's too much hassle and the result is only better slightly. Spam actions even with phones is not what they used to be.
One model = one page.
They normally get banned after a few actions. Since I'm registering them on the same useragent and also the proxy is from the same location - they are far more likely to live. Normally you can't get such results with accounts you get off of marketplace, unless you are working with private suppliers who disclose you how they are making the accounts so you could adjust the set-up to mitigate bans.