It seems that 8 GB of RAM is at least in part responsible for the bottleneck, no matter what you put in `dbcache`.
Your first reply has a clue on this.
It may say 7.2Gi "
available" memory, but that's after the 7.2Gi "
buff/cache" are written to your disk.
Given that Core is already using the drive extensively, increasing the database cache doesn't help as much
since it will otherwise require your OS to swap more memory, further increasing the disk usage.