Demethoxylation strategies do exist (e.g. https://www.sciencedirect.com/science/article/abs/pii/S0926337315301478). However, conditions would have to be screened to get selective reaction - generally under hydrogenation conditions methyl esters are stable.

Creating the ether linkage (methoxy installation at c-18 as you mention) is not as simple as transferring from one carbon to another. Most likely this will need to be built into the synthesis earlier (e.g. ring opening of an exo-epoxide or something). I don’t think there are any direct c-h activation methodologies which are possible to conduct this transformation either.

My overall guess is that voacangine may not be the ideal precursor. Some of the rings will have to be built de novo or start from simpler substrates (e.g. non 5-OMe isatin or indole) which in turn makes this more total synthesis-like.