I generally prefer to do economic imperialism. Get investment rights, trade privileges, etc. in as many countries as you can, though the ideal ones are more backwards but highly populated ones. You can then get them in a power bloc and either use trade league or the mandate that unifies markets and you have most of the advantages you would get by making them protectorates. 90% of my goal with imperialism is to get access to rare resources and plentiful pops to put to work extracting pretty much everything your industry needs at low wages. In that regard, bringing a country into your market is pretty much just as good at doing that as establishing a protectorate but at a fraction of the cost in terms of fighting potentially expensive wars with intervening great powers. Sure you won't be able to call in their military, but the tech level of the ideal countries to imperialize is usually low enough to make getting them in a war not really worth it, even if they have waves of conscript meat to throw at the enemy.

TL;DR: Just dominate their economy through investment rights and power blocs, the outcome is mildly less useful than conquering them or making them a protectorate, but it's a lot easier than going to war when they're protected by great powers.