I quote BASIS_MOLOPT file:
# Usage hint : 'NGRIDS 5' (section &MGRID) will deal more efficiently (2x speedup) with the diffuse nature of the basis.
Due to that, don't bother too much with REL_CUTOFF. Just comment or delete the line and let CP2K use the default value.
You indeed have to study the convergence of the CUTOFF. I'm not bothered by 600 Ry. But you should assess this value in the range 300-1500 Ry for example, with a variable-cell optimisation of a CuC crystal at ambient conditions. The energy and volume should converge for a certain value of CUTOFF.
I'm not sure "saving" on EPS_SCF is a good idea. Did you assess 1e-5 was okay? I would say at least 1e-6.
On the other hand, ALPHA = 0.1 is small. How does 0.2 or 0.3 behave?
My 2 cents.