Outstaffing consists in hiring remote developers via an international staffing partner and integrating them either in your main team or as a secondary team. Now if you want to get the best of what this model has to offer, here are the rules:
More software developers available makes recruitment faster and rates lower. But do not rush or get greedy. The real opportunity there is to reinvest some of that time and money saved into higher standards. A real expert is often more productive than two average engineers.
Consider also the opportunity to get a wider team with complementary specialties, for example instead of just 2 fullstack european developers, get 1 frontend developer, 2 backend developers and a QA.
Do not just build your own external subcontractor. Access to the team is the one big difference between oustaffing and outsourcing.
Managing your remote team as if it were in-house enables you to implement agile development best practices and have a clear view of who is actually working on what, what the issues are, and where the project is headed. That is also the best way to have a really motivated team.
You want your developers to have motivation and energy, and to stay with you for years. They need the same things as everyone for that.
First and foremost is working with them and integrating them. This also means building relationships, getting to know and trust people (which is a very rewarding aspect of outstaffing by the way).
Next come perspectives : people won't stay with your for very long if they have none. Leading a team, mentoring juniors, or just developing new skills for example.
Finally, paying rates reasonably higher than market is very efficient, but will not dispense you from any of the above.
Working with your developers, managing and motivating them is already a lot of work. What you should not do on top is trying to become your own outstaffing agency. Believe me.
You'll start by wasting a ton of time chasing candidates the wrong way (eg on linkedin or upwork). Then you'll get ripped off in negotiations once or twice, discover your bank does not support payments to Ukraine, and your HR and lawyers will try to dissuade you from doing something they don't know how to manage.
But the real problem here is that it will defocus you from your real job. You're supposed to build software. Let us handle the rest and setup a great team for you.