I suggest adding these conditions:
1. When a new sof is played, it absorbs all other (indeed can be only 1) sof already present on your field. The new sof has their combined hp and can turn to blackhole instantly when it exceed to critical value (I suggest reduce it to 40).
2. Using the ability delays it by 1 turn.
These prevent fractal/mitosis.
But I think scaling down the hps (initial/gain per use/maximum) is a bad idea since one can keep it reaching max hp much more easily (eg putting right on amount on poison then quint it).