Ok first of all yes 1.13.2 is not a good update since they installed new systems which were not performance improved at all. I agree, but the memory requirements you suggest are not even closely representing what 1.13.2 will take. It will take the same amount of memory regarding active memory usage.
Since most of the actual memory usage comes from Models & Textures which by the way forge causes not Minecraft itself (because minecraft unloads the textures from ram while forge prevents that (see texfix mod).
On top of that 1.13.2 modding wont be such a huge thing, because it is like the 1.8 version or 1.9 because 1.14 is so close and besides some core engine rewrites it is not a lot different, these rewrites do not affect mods that deeply since its more the internal stuff that changes while the external stuff stays the same.
So expect 1.14 to perform better on low end machines / server compared to 1.7.10 or 1.12 especially since the light engine and a lot of other things got removed from the ServerLogic itself.
Yes 1.13.2 was a mess of a update but 1.14 is the long awaited cleanup on the game itself.
And i do not think that this is solving everything but it should improve performance drastically compared to 1.13 and a lot compared to 1.12 (or 1.7.10)
I mean in 1.13 was multithreaded worldgen added, and if mods can use that actually (and do not fuck that up) then expect things to run a lot faster on servers. Especially with the 1.14 cleanup