The blend door might not be swinging properly, to force air through heater core.
Not sure if chevy uses vaccuum operated valve to stop/allow coolant flow to heater core, but there is likely either mechanical or vaccuum valve on the heater hose plumbing somewhere. Look at rockauto of heater valve under the heat airconditioning section as to what it looks like.
I just had to change heater hoses on my Dodge, the old valve was partially occluded. I left the valve off for now and plugged the vaccuum line to it, whilst I wait for new valve to arrive.
I flushed and backflushed the heater core. I did so about a year ago too and much more crap came out last year but some did come out this time too. last years flush I got 25f more degrees at output vents, did not bother collecting data this years flush.