Proof Mind is Fundamental and Matter Doesn't Exist

[Angrboda]

Premises 1 and 2 are a classic argument from ignorance, and hence the conclusion  that solipsism is possible doesn't follow.

is that so? the first two premises was specifically to show it is by definition it is unreasonable to believe solipsism is impossible. that means to our epistemic knowledge (for all we know and could possibly learn) solipsism is possible. as such, it is then also reasonable to conclude solipsism is actually possible. you can try to argue we can't use epistemic knowledge to make metaphysical claims and therefore we should be undecided in those terms... but then that would also be to divorce metaphysical knowledge from knowledge you could attain. thus it would not be reasonable as to say 'we can't know about the metaphysical' as that itself is a metaphysical claim that you likewise could not claim to know. thus the only reasonable alternative is to presume we can know of the metaphysical and therefore solipsism is possible.

But you aren't using epistemic knowledge to make a metaphysical claim, you are using the absence of epistemic knowledge, ignorance, to make a metaphysical claim.