Rule of thumb: minimum feature size = 2× wire diameter + kerf margin. For 0.3mm wire and ~0.5mm/side kerf on EPS, that's about 1.6mm minimum. Trying to cut a 1mm slot will fail — the kerfs from each side overlap and the slot becomes a single melt.
For really fine work, switch to a thinner wire (0.2mm) and accept the breakage risk.
For really fine work, switch to a thinner wire (0.2mm) and accept the breakage risk.