Without seeing any images, this is an appallingly difficult thing to try to provide advice upon, because it's impossible to guess why you're not happy with what you've managed so far.

Subterranean silver mines in ancient to medieval settings (presumably what you'd be aiming towards for a fantasy setting, and assuming you're not opting for some form of surface/open cast mining, which would be an alternative) essentially looked a lot like mostly narrow, often straight, low caves. The nature of these will be heavily dependent on what state the silver is in. If it's in the rarer metallic veins, the narrow, tunnel-like caves would be appropriate. If it's in the commoner form found on Earth, an ore combined with lead or copper, which needs further processing to extract the silver, larger areas of higher-grade ore might be opened up as a cavern. In addition, processing facilities would almost certainly be constructed on the surface nearby for the latter ore-type mines particularly (or perhaps underground where undead workers are being used, as here), which would make for a larger, more varied complex overall.

The mine needn't require vertical access, as many early silver deposits were extracted simply by starting to dig a tunnel into a hillside more or less horizontally. That would be an option for a more 3D map, however.

I don't know. What you've done has the advantage of real clarity, and to an extent, it reinforces the "area of the unknown" the events took place in, given the river valleys were the only places relatively rapid travel was possible.

For the zoo map, how closely are you wanting it to match the "real" zoo map? A modern mapping style might work better than "straight" DD3 to match the simple block-colours scheme, for instance. Also, you might be able to find a free font with suitable animal silhouettes you could rework as symbols (or just use the font as-is, if it'll be a one-off map). Those would match these quite well from some free animal fonts I've come across.

I think this may be Remy's blog article you mentioned, Sue. It includes notes on the RGB Matrix Process, at least.

Given the blog article uses an overland example as its RGB Matrix Process test, I'm guessing it will work with that at least (can't be sure if the tree symbols have been affected or not in the blog sample images, however).

A quick test, slapping a greyscale RGB Matrix Process Effect onto one of my own overland maps (whole map setting) suggests it does indeed convert all the symbols to greyscale, while adding a few random choices of dungeon symbols to it, suitably enlarged, shows they too convert to greyscale without problems. Obviously, this is hardly definitive for all possible symbol sets, but it doesn't suggest there's a problem with what I've checked so far.

Actually, any colour version can be readily converted to greyscale simply by applying an appropriate RGB Matrix Process Effect to the whole map, so that would be win-win!