I think one of the most important things to remember when evangelising online is that we aren't responsible for changing someone's heart — that's the work of the Holy Spirit. Our role is simply to witness faithfully to Christ and to speak the truth with charity.
Jesus didn't tell us that everyone would accept the Gospel. He said, “If the world hates you, keep in mind that it hated me first” (John 15:18). So encountering hostile or militant responses shouldn't necessarily discourage us.
At the same time, I think there's a danger in getting drawn into endless online arguments. We can become so focused on proving someone wrong that we lose sight of the person we're actually trying to reach. St Peter tells us to “always be ready to give an answer” for the hope within us, but to do so with gentleness and respect (1 Peter 3:15).
Sometimes the person arguing with you isn't the person you're actually reaching. As you say, there may be others quietly reading who have never really considered Christianity. A calm, charitable explanation of the faith may stay with them long after the argument itself has disappeared.
I'd also say don't underestimate the power of simply sharing why you believe. Personal testimony can sometimes reach someone where theological arguments don't. And don't feel that you have to respond to everything. Even Jesus sometimes simply walked away when people were unwilling to listen.
Ultimately, we plant the seed; God gives the growth (1 Corinthians 3:6). We should proclaim Christ boldly, but without becoming combative. Truth without charity can push people away, while charity without truth isn't really evangelisation.
And perhaps most importantly, pray for the people you're speaking to. The aim shouldn't be to win internet debates but to help people encounter Jesus 🙏✝️