1) But convenience is objective! (It is not convenient to sit in a chair that is hanging upside down.) Which is why matter, even if it is seen as a "convenient" mental construct, is still objective.
2) Materialism and "objectivism" are closely tied together. Mathematics consists of abstractions (or models), and those reside, in particular, in our mind; being "convenient", these abstractions are therefore reflections of what lies outside, and so - almost by definition - the objective world itself, matter, cannot possibly be built from math. (Besides our mind, "abstractions" can also reside in other, inanimate, objects; regardless, this still means that some part of matter merely reflects, to a degree, some other part).
2) Materialism and "objectivism" are closely tied together. Mathematics consists of abstractions (or models), and those reside, in particular, in our mind; being "convenient", these abstractions are therefore reflections of what lies outside, and so - almost by definition - the objective world itself, matter, cannot possibly be built from math. (Besides our mind, "abstractions" can also reside in other, inanimate, objects; regardless, this still means that some part of matter merely reflects, to a degree, some other part).