Im looking for solutions they avoid me writing a novel-length set of instructions as well as zero/minimal trust in 3rd parties.
This is a tough choice. Either your family members learn how to deal with wallets and private keys, or they should find a trustworthy 3rd party. Maybe there is another option, but I don't know about it.
You can leave this choice to them actually. The main thing is you keep the private keys (to all of your coins) in a deposit box or somewhere that your family knows about.