My big tank has a twin skin 12mm + 12mm base panel, it is full DIY and has been alive for several years, you'd get away with a repair panel and silicone. Fortunately the crack runs out on a small corner. I've made repairs of this nature on smaller tanks.
Plenty clean, and clean again. A good bead of silicone. Should be fine. If the repair panel is of a suitable size, you should be able to refit the overflow box.
Just be sure you clean the area thoroughly.
You get the panic of will it hold with any assembly of a tank, I know because I've made several. Even the small ones are a cause for concern. The bottom will only fail or flex if the stand is inadequate, in which case nothing you do would be good enough.
I would make the panel at least 3-4" over the edge of the crack, leave a 3mm gap at the sides and rear, fill with silicone, gently ease/jiggle it back into the corner (lose 1mm of gap)
To ensure silicone has bedded properly. Finish off around the front edges or repair panel in good time, ie before the silicone skins over.
Use glass of an equal thickness.
(Glass will only shatter if it's toughened)
Clearly this is normal (float) glass.