How can we reconcile the Hadith, "Were it not that I would cause hardship for my Ummah, I would have ordered them to use the Siwak," with the Hadith, "When I command you to do something, do as much of it as you can"?

The required Sharia rulings are of two types: obligatory (wajib) and recommended (mustahabb). The obligatory must be performed; otherwise, its ترك (abandonment) incurs sin. The recommended does not necessitate performance, so if one abandons it due to a tolerable hardship, there is no sin upon them. The Prophet's (PBUH) saying: "If I command you to do something, do as much of it as you can," is among the comprehensive statements and provides excuse for one who is unable to perform the obligatory duties (fara'id) or some of their pillars or conditions. In such cases, one performs what is possible and is excused for the rest.