CaptVee has it right.
CNC machines are lightly loaded structures, typically loaded to about 1-3% of strength. 10-50 N kgf /uM of movement.
Ie a 1-5 kg push must deflect only 1 micron.
Rigidity is everything.
Mass is usually everything.
Cannot think of a single example where its not.
All the best well-known lathes (ie Monarch10EE,Hardinge HLVH, Schaublin) are all heavy for their volume.
Mass absorbs vibrations (but may not improve strength. This does usually not matter, because the loads are so low vs ultimate strength).
Your build looks very good, ime, imo.
It will likely leave excellent finishes with finishing cuts, due to excellent vibration absorption.
Granite = 10x mild steel by mass.