When is it time to replace my trainer tire

I wouldn't hesitate using a tyre in that condition on the trainer, in fact that's exactly the sort of condition i might expect to start using a tyre on the trainer.

The small cracks likely mean the rubber is deteriorating and may be less grippy, but that's of little worry. It's very unlikely there is substantial damage to the actual carcass of the tyre.

Finally, the absolute worst case scenario is a small split begins, the tube pokes through and bursts - however positioned where they are there is very little chance of the beads becoming unseated. So maximum damage is the embarrassment of explaining to people you got a flat on the indoor trainer.