I think it is unlikely that luci uses (or frees if removed) enough memory to make a difference considering all the low memory devices it has run on over the years, but I’ve never bothered doing a test with and without it in the firmware to see. Quickly, I looked at memory in use in htop before and after logging into my gateway with luci and didn’t see a change in memory use (to the nearest M anyway). But that doesn’t say anything about what might be preloaded into memory.
I also doubt that LuCI makes a differences. What does work for me, using the MX4200v1 as a wired AP, is a regular reboot every day (via cron).
Pretty sure DD-WRT uses NSS (by default, anyway, you might be able to turn it off).
Why do you want to avoid NSS? All things being equal, sure, I’d rather use “vanilla” OpenWRT, but NSS leverages capabilities of the SoC to give you better device performance. I and many others use the AgustinLorenzo builds.