Facebook campaign strategy: several campaigns with few countries or few campaigns with several countries?

We’ve found that the best strategy is to segment countries by LTV tiers in separate ad sets(eg separate out all countries with LTV > 4, LTV between 3 and 4 etc.) - while making sure that each ad set/segment does not have less than 1 million users.

If some countries do have low ROAS numbers, it’s generally not optimal to remove these countries because this can reset the learning phase(as you’ve pointed out).

however if you really find that a specific country’s ROAS continues to be bad, it’s best to start a new ad set with the entire segment but without the low-performer.

You certainly can use adjusted bids by countries using bid multipliers. -> https://developers.facebook.com/docs/marketing-api/bidding-and-optimization/bid-multiplier/

You will need to be whitelisted or use a tool like Smartly to use it, though.

If you can't do that, I'd recommend grouping countries that have similar CPIs and conversion rates if you're optimizing for CPI.

Ideally, you'd want to optimize for Purchases / Subscriptions / ROAS and let Facebook do all the hard work. That way you can group any country with any country because you really only care about your final metric, not upper funnel metrics like CPI.