This lines up with a book idea I've had for like 20 years. Crazy!

Don't wait to write sci-fi I suppose! Life may catch up, haha.

I'm trying to work out how the fungus evolves to grow its food source by causing radioactivity increase?

It can concentrate radionuclides, but the step function after inducing some criticality is likely to cause reproductive difficulty (stopping fungus evolution).

Plus: heavy metals combined with organics have a tendency towards being nasty poisonous

Idea is nothing, execution is everything.

Just write it if you want to.