-No, this is the best site to be on. Participate, post your art, apply to jobs when they're posted here, and you'll get a trickle of income, it will be 'part time' still but everything else you do will depend on your goals long term. As of present the Cartographer's Guild is fairly unique in its focus. Your other options would be to join forums relevant to the specific hobbies or careers that you may target as your customers. Authors, for example, so participate on a writing website. RPGs, so start playing them and getting to know Dungeon Masters. Etc. Identify a potential audience and go to that community, not necessarily a mapping/artistic community.
-I have never cold called. However, I will talk about my work if it comes up organically in conversation, which is why I recommend community participation in a place a buying audience would be found. I recommend not exactly what you say here, but to always ask your clients to refer new clients.
-No, there is no such thing as a cartographer agent and anyone who pitches that to you is a scammer. There's only networking or advertising. Agents don't deal with single pieces of art like this, and it's extremely rare you'll work with one as a customer, it's far more likely to be an independent author who hires you than an agent, or a publishing house. When I got a publishing house client I asked them to refer me to other Canadian publishers they might know who need maps once we were done with our map...
-Selling assets / creating passive income is a vital component of making a career out of this, I feel it is possible to get enough individual clients, but it helps a lot to have sales from a storefront. You do watercolor, so make watercolor texture packages, I buy those so often. RPG components are fairly saleable right now, I would recommend it to anyone seriously intending to be a cartographer, it'll get you a good step in with meeting future clients if you become involved in the RPG community as many of those fans of the game go on to write modules that need maps. Or make maps to sell on DriveThruRPG.
-Make a portfolio that is self-hosted and not bound to any service changing with time.
-Put something on there that isn't your art, and is attractive bait for someone who is of the target audience you aim to capture as a buyer. Like free textures that someone has to sign up for an email letter to receive, that's one I've seen a few people do. I have map making brushes on DeviantArt.