Is my monthly commitment of a quarter of my salary for a good cause, given other financial obligations and an upcoming marriage, considered extravagance or reliance upon Allah?

Spending money on good causes is not considered extravagance. However, if it conflicts with a marriage project, it is better to prioritize furnishing the marital home over charitable contributions, because marriage, when there is desire, is better than supererogatory acts of worship. This is also because it protects one's self from temptation, especially when living abroad. Furthermore, spending on oneself is a form of charity for which a Muslim is rewarded if their intention is pure.