All I can add is that we also had to do this (several years ago) and we just did our best to give them whatever they asked for - jumped through every hoop they gave us,and after the 18months finally had our son placed with us!
From our group, the types of volunteering people did were to go into schools and read with children, brownies / scouts, local playgroups, local baby groups.
It is an utter pain to try to fit into your working week and I agree with the others here who have said nothing will prepare you - you don’t even know what age the child you adopt will be!
My impression is that they have a system, and it is not for you to question it! They usually are oversubscribed with people trying to adopt and if you can’t get on board with it, especially in the early stages, then they might not let you progress. For us this was certainly the case - we were not allowed into stage 1 until we had some experience with ‘young children we didn’t know’.
I think you might be setting yourself up to fail if you start to challenge them? If you do proceed with Coram, I do think the baby groups etc will start up again and you will have your opportunities. You could check how much experience they want - provide them with your plan before you start and see if they think it is sufficient?