and the temp fuse with 100° looks a bit low...some tolerances and some ABS printing ( 100-110°) and the fuse cuts off..and im sure in that moment you forgot that there was a temp fuse 😉
300W seems really low...in my Dleta i use 300W für a Ø 300mm 8mm cated aluminium bed and its ok..
External temp controller is ok..but not so comfortable, because you have input the wanted temp manually..another thing to forget before a large print..
Most uses an SSR as a slave and the duet as controller... and pls dont forget to check the electric..i bought my heated bed uses..and the former owner have used it without ground on the aluminium plate..