Thyme doesn't spread real fast but it does spread. Around here, we get a winter every few years that kills it, though, and I have to start over anyway.
I used an old double-tub clothes washing setup for a planter years ago, but during the real hot part of the summer the plants would wilt and die back even with 2 waterings a day. That tank of yours has enough mass that the roots don't get hot. And up off the ground like that, bet you don't get any weeds.