I know about window managers and how using them will reduce the memory usage by system a lot because they are less bloated etc.

Ehhhh… I think it’s more “not using a curated general-purpose DE”, rather than “using a WM”. All graphical systems include a WM, and a DE in some senses is more of a concept or category than a concrete thing. The choice is whether it’s one you cobble a DE together yourself, or use a pre-configured, curated one.

Many people use stand-alone WMs and then create their own DE, but quite a few of us put the WM of our choice within existing DE because we want the WM but have no interest in re-inventing all those DE wheels (and/or have >4Gb memory so the “bloat” is not an issue). In my case it’s i3 on Gnome via gnome-flashback.

Curated DEs do tend to use more resources - typically mostly memory - partly because they tend to be comprehensive for diverse users. Rolling your own minimal DE for your personal needs can often be lighter weight. If you have a very constrained system then it can be beneficial, though that circumstance is more and more unusual these days when 8Gb of memory is often considered “minimal”.

The main reasons for making your own DE is to do things exactly the way you want, at the expense of having to do it. Beware though, there will be various helpful features of DEs you may not realize you appreciate until you have realize you don’t have them. E.g. what happens when you plug in a USB drive? Nothing, by default - a DE usually manages that. SSHing into servers a lot - a credentials agent is nice - better add one of those…

A lot of rolling your own DE is months or years of “oh yeah, that is a useful thing to have; I need to find tools and configure them to do that”. Conversely, dropping your WM of choice into another DE is often a case of “huh, that happens automagically; nice!”.