Should I make up the Ishā prayer that I performed alone outside the mosque in order to catch the congregational prayer, even though the mosque was not full?

If you are aware of the Imam’s transitions through seeing him, or seeing the congregants, or hearing the Takbir, then your prayer is valid and you are not required to repeat it. The Hanbalis stipulated, for the validity of praying behind an Imam outside the mosque, seeing the Imam or someone who sees him. The Shafi’is permitted praying behind an Imam with conditions, including that the distance does not exceed 300 cubits and that there is no obstruction preventing passage. The preponderant opinion is that your prayer is valid even if these conditions are not met, based on the Maliki school of thought, which permits praying behind an Imam whenever it is possible to know his transitions by seeing or hearing.