Thawabu comes from Arabic ثَوَاب (ṯawāb, "remuneration").
Quite apart from the fact that the vocabulary isn't really a priority in a beginner's course in Swahili, I'd also say that it's more naturally English to say "Spiritual reward is good".
What is the context behind this? What kind of spiritual reward?
According to this site https://glosbe.com/sw/en/thawabu, it means "gift (especially from God)".
Example: Why is it proper to look forward to the reward of everlasting life? -> Kwa nini inafaa kutazamia kwa hamu thawabu ya uhai udumuo milele?