If this DE has low resource usage and working freedesktop screensaver protocol implementation and idleness implementation but doesn't force me on their toolbar and window manage I'll take it immediately.

Lxqt is the closest to allowing that because it wraps standard tools when possible, but this wrapping of standard tools also means that those tools don't really work.

For example, I use slock for screenlocking, but there is actually no working third party freedesktop screensaver implementation that doesn't tie you into their DE.

Xsecurelock seems to have hacks for it, but it can't even do something as simple as just showing an image without breaking with the wrong window compositor.