With a clean install of Ubuntu 18.04, am I able to uninstall GNOME and have just a window manager?
This sounds really stupid since a window manager probably relies on a DE, but no risks taken asking.

I do not know what do you really mean by that, but I think yes, you need to run startx. Or just install lxde (or something not-so-fancy one).

Or even without a WM.

FYFI only here

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No problem! Just install a suitable wm (icewm, fluxbox, openbox or one of the tiling wms like awesome or ratpoison). Then set up your display manager to load that instead of gnome.

Uninstalling gnome is a bit more tricky because desktops like gnome come with their application suites as dependencies. If you get rid of the desktop metapackage, the apps disappear too because they weren't explicitly requested for installation. Of course you'll get a list of what's going to be removed. Just write down the ones that you want to keep, abort the uninstall and install those packages explicitly. Then remove gnome. Or alternatively, remove gnome and reinstall those packages afterwards.

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There would be a lot less cleaning up to do if you just replaced Ubuntu with something like AntiX or SalentOS which comes with a wm as standard. For example, a desktop creates your menu when you log-in, but the wm will expect a menu file, so you have to write it. There's also the possibility that you mentioned that you have useful tools which are written on the assumption that Gnome will be present: I remember with Fedora that none of the configuration tools had any help unless you had yelp to display it.

You are potentially in for a world of trouble if you uninstall GNOME seeing as it is, as far as I recall, gratuitously coupled with systemd and there are various applications which will depend on it. You will be much, much better off just selecting another WM as default.

Try one of the 'lightweight' versions of Ubuntu, rather than trying to rid yourself of Gnome, as it will cause you less trouble.

But you could also look at a different distro, such as AntiX or MX Linux, or many more that come with just window managers, rather than full desktop environments.

It might be worth pointing out that you don't need to uninstall gnome to be able to test different desktops and/or window managers.
As others pointed out, install a window manager suitable for standalone usage (openbox and fluxbox are good candidates) and from the login screen start a session in said window manager.
If you like what you see, you can still look at "cleaner" options such as installing a different ubuntu flavour or a different distro altogether.