The only complicated part is the trust for your niece and nephew. Trusts are never straightforward as they have to be administered.
DIY wills are perfectly good if the basics are followed, but in your case it would be best to get your solicitor to do it.
Before the appointment, make sure you have all the relevant details to hand - full names and addresses of all beneficiaries, including the charities, all the property details.
Write down precisely what you want. It will save time and money at the solicitors.