You would have a member group (say “members”) that receive the experience that contains ads. You also have another group for your subscribers (say “VIP Members” or just “Subscribers”). Using the subscription rules, if someone purchases a subscription they’re moved into that new subscribers group automatically that does not have the ads included. If the subscription is ended, it can be even configured to automatically move the person back to their original group.
Take a look at the following guide for more details:
This is doable, but not done automatically within the software. You could certainly add the HTML code to do it within your theme relatively easily.
In addition to subscriptions (which are recurring), you can also make one time sales such as what you described.
See the following guide:
Based on the traffic you were describing, unless you had a very specialized niche that had valuable ad keywords, ads lately are not enough to live off of alone most likely. You need huge traffic volumes to really be truly profitable. But it could certainly augment an overall monetization strategy.