Garbled text (fonts?) in display managers/desktops

I recently installed FreeBSD 11-stable onto a Dell Dimension 2400. This is an older desktop, circa 2003 or so. So by today's standards it is underpowered, but at the same time it's old enough that I would think the hardware should be pretty well-supported. In any case, it's a test system for me.

While I haven't had any major issues on the console, I've had one frustration after another trying to get a reasonably working GUI. I've tried 3 different display managers: Slim, LightDM, and GDM.

Slim: If I add slim_enable="YES" to /etc/rc.conf and reboot, I will get a black screen. But if I take that line out, boot up, and type "startx" from a command prompt (with "exec mate-session", for example, in ~/.xinitrc) and start up the desktop environment then logout and type, as root, from the command prompt "service slim onestart", the Slim display manager will display properly. I can then login without any problem.

LightDM: If I add lightdm_enable="YES" to /etc/rc.conf and reboot, I will get the LightDM login screen, but the dropdown menus will have garbled text. I can then login without any problem, although I have to guess at what I'm choosing from the dropdown menus since I can't read the text.

GDM: If I add gdm_enable="YES" to /etc/rc.conf and reboot, I get a properly displayed login screen, but am unable to login to any desktop environment because it apparently will not release the display to the desktop environment.

I also installed Gnome3 from pkg, and I can start up Gnome3 from the console if I type "startx" (with "exec gnome-session" in ~/.xinitrc), but some applications, like MATE Terminal, will not start. Also, if I open up a terminal like LXTerminal, text will not display at all in the terminal window. If I open up a program like the GNOME system monitor, the text displayed will be garbled and unreadable.

There are obviously multiple issues here. I'd like to be able to use SLIM, but the black screen makes it a no-go. It seems odd that it will only work properly after a successful desktop login/logout but not from a fresh boot. And why am I seeing garbled text in some situations? Could this possibly be a problem with fonts?
How about trying the default twm and seeing if you have any issue with that. It doesn't depend on anything else, is installed by default, and is pretty lightweight to boot, so-to-speak. It might help eliminate some of the issues... or not.