Metaphysical Foundation of Science:
✧ The external world is real and knowable.
You are right. I do not share the same "metaphysical, assumed truths" you listed above. Indeed if you deny the existence of objective truth altogether. The first of the metaphysical assumption of science above then you are in the orbit of a very different belief system.
Definition of Nihilismhttps://www.merriam-webster.com/dictionary/nihilismA : a viewpoint that traditional values and beliefs are unfounded and that existence is senseless and useless
B : a doctrine that denies any objective ground of truth and especially of moral truths
I cannot disprove nihilism only point the the various reasons it is a poor choice.
If you have time I recommend the following essay on this topic by Bruce Charlton.
Metaphysical AttitudesTake Care