When bidding on CPM traffic through a self-serve RTB DSP, what are the most efficient strategies for initially choosing sites/apps, and setting a CPM?

The answer depends on your situation: You could be a start-up that is more concerned with not blowing tons of money, or you could be in a rush to be done. In both situations, your objective is to find inventory sources that work quickly and then massively scale into those inventory sources. I am going to assume that the goal is to be efficient. In this case, I would start with relatively few inventory pieces and a pretty low bid.

The goal of your experiment is to determine what inventory DOESN'T work while spending as little money as possible. By concentrating your spend on a few pubs, you can get to significant results quickly. If you spread out your spend, you won't generate enough clicks or backend actions per inventory source quickly enough to determine if one is actually working. (In my latter user persona, you can simply spend gobs of money to get significant results on lots of inventory quickly.)

So the moral of the story is that you want to get to results, you have to scale the number of pubs you work with to align with the amount of money you are planning to spend. You would like to, within a day or two, rule out bad inventory and focus on new inventory, or alternately, make massive investments in working inventory to print money for yourself.

I would be happy to do a call to further discuss.

Your first instinct is what I would go with as well. As an ex affiliate marketer I can say is that in the beginning of a campaign you are going to lose money most of the time. But in return you are getting all this valuable data of where your campaign is converting and where your campaign is not converting. With that data you make your campaign profitable. You can go with the other way by adding a few individual apps but I just see that as time consuming where you can test out all the traffic right off the bat in the beginning.

It really depends on if you're optimizing for cost or speed.

If you have time constraint and need to move fast you basically pay for it with bigger upfront budgets. You will definitely "waste" more of your marketing budget going this route but you will also get you to your goal much quicker (basically, you're buying the data you need to scale).

If you don't have a large marketing budget, then you really don't have an option but to start small and expand.

In both of these scenarios it is critical to instrument your app properly so you are getting good data on conversion rates, engagement and LTV. If you're not sure that you are collecting this data properly I'd do a smaller test campaign before going all-in on a big marketing push.