I have a system somewhat weaker than yours, with AMD N36L and 16 GB of RAM, maintaining nodes of a similar size to yours. CPU not strong enough for zram-tools.
As I wrote here, -I 128 reduces metadata size. I expect ~1 GB of RAM to be enough to cache 2 TB worth of piece files with this setting. Requires a fresh file system.
LVMcache is a good thing.
BTW, I suspect after recent changes disabling synchronous writes that btrfs might actually be viable now.