J’ai fait tremper les pruneaux. doesn’t mean you had someone doing it for you but that you had something doing it for you:

I put the prunes into water for them to be soaked.

Similarly, j’ai fait griller le pain means:

I put the bread in a toaster which toasted the bread.

The most natural translation of these sentences seems to be:

I soaked the prunes

I toasted the bread

Unless it would have been done by someone else, I don’t think it would be idiomatic English to say:

I had the bread toasted (by the toaster).

I had the chicken roasted (by the oven).

while in French, it is usual.

We are probably less likely to say the following sentences, but they are fine too:

J’ai rôti le poulet.

J’ai grillé le pain.

Moreover, your suggestions do not match the French sentences tense. Both

I’m having the prunes soaked

I’m getting the prunes soaked

are taking place in the (continuous) present while "j’ai fait something" happened in the past and is done now.