It sounds to me like you have two things you care about here: customisation, and community. I don't know the lengths to which Tumblr is customisable, but for a one-person operation it sounds like it's fine. I'll point out that if you self-host Wordpress you can do pretty much whatever you want with it, and plugins make it extra-powerful. But that's beyond what it sounds like you want. It may pay to get a domain and have it point to your site, either way. This way people who follow your work have one place to go to find you, and that place won't change even if you change platforms. The main problem I see with a Tumblr-powered blog is that Tumblr hosts it all, which means that if they go down/go out of fashion in 2-3 years you have to pull everything out and insert it into your new blog engine. Also, you have a bunch of followers outsider of Tumblr, a decent proportion of which you're going to lose when you switch, I'm guessing. (Incidentally, doesn't hurt to keep a big "I'VE MOVED HERE" sign on your wordpress blog for six months, in case people spot it in google or the like.) Re: exposure. I'm guessing that involving yourself in writing communities (both local and on the internet) would help? The problem with the internet and its low barrier of entry is that there's so much noise out there of everyone spouting their thing. If you have a community (as you would do with Tumblr) people can find out about your stuff slowly, as your friends reblog you being clever etc.. I tend to feel that's a better way to gain followers than spamming everyone you know with links. However! I am generally not good at promoting stuff on the internet: it's not something I've attempted much of nor studied. So take that all with a grain of salt.
I like your ideas and you bring up a good point about Tumblr possibly (and most likely) going out of style in a few years. I was thinking of setting up a self-hosted WordPress blog for myself, that will serve as the main portal for all my various projects, experence, examples of work, blog, thoughts, etc. I was thinking of just linking to the WordPress blog/Tumblr. Now I'm starting to think that I may as well just publish all of the writing on a Louder Words section of my website. Also, how long did it take you to set up your website? I really dig the style, and the source code looks fairly simple/clean/manageable. Any recommendations for that process?
Oh, that thing? It's grown, slowly. It uses jekyll on the back-end, which is an interesting/masochistic way to make a website. The main process was: put something on the internet, add content, work out what sucks about the layout, fix that, repeat. There's a lot to be said for shipping early, then refining the product. I think I've been doing that process for six to eight months? It's been on-and-off, whenever I feel the need to move something around or push something out of the way. (There is actually a slight redesign in the works that I need to get around to final testing + uploading at some point). The layout itself is based on my looking at websites, going "that looks pretty/striking/better than my website", bookmarking it, and then going back later and working out what makes it tick before replicating it on mine.