Sorry, but that's just not out of line in the modern iPhone world. What's not adequate is owning an iPhone with only 64GB of Storage. When you bought your iPhone, you bought one with the lowest storage level available. People don't seem to give enough consideration to the storage options when buying an iPhone. Today, Apple only sells new iPhones with 256GB as the lowest storage level. I now only buy new iPhones with 512GB.
As it relates to System Data, the good news is it will fluctuate and if space is needed, it will automatically lower. You cannot manage System Data as it is comprised of Cache and files and data which cannot be categorized, but helps your phone operate.
I would, if I were you, give serious consideration to upgrading to a new iPhone with much more storage. No one has ever come on this forum to say they have too much storage, but many come here complaining because they don't have enough. And every year, our storage needs will grow.