Actually, if you build GTK3 yourself, it doesn't need anything more than a current default GTK2 distro package does, but you'll lose features like colord support. My experience is with Gentoo USE flags, but I believe FreeBSD ports has options for packages that do the same and are the original inspiration for USE flags, so one should be able to use a lean GTK3 and GTK2 on FreeBSD.
When GTK3 and even Qt5 go user-unfriendly and dev-unfriendly ways, it's no wonder there's many immediate mode ui library projects that are popular or actively developed Haskell FLTK bindings.
My point is that a lean build of GTK3 doesn't require more than GTK2 right now does. In the port linked to, libepoxy and libcolord can be avoided. And when you do that, it's down to what GTK2 depends on in its default build config on most Linux distros. If you could disable ATK, you could shave off some more, but GTK3 doesn't build without ATK.
Qt5's base memory overhead is big (minimum 25MB for the simples hello world window you can create). The modularization of Qt5 is a good idea, but it also makes use on Linux distros a pain, because it's too many modules, like the mess some Linux distros create when they split core Perl into a gazillion packages.
Just use Tk. Or wx. They're both well supported, cross platform, and if you use ttk, you can even have your app look good. And native.
For WM, I use i3. If you don't mind tiled, I'd highly reccomend it. It's great, the docs are great, and the config file is easy to understand, so after the base setup, you don't have to worry about it.
XFCE, TWM and sawfish are pretty cool, if you're not into tiled.
Yes, Tk just works for gui and is underappreciated.
wx's default backend is gtk (2 or 3) and they have an experimental Qt5 backend now, which is unsurprising given GTK3's state. So wx doesn't help, unless you can make the Qt backend work (Qt5 is so modular that it's often a pain).
GTK3 drags in way too much of gnome for comfort, but I don't want to drop all GTK3 apps, so I have to put up with it.