Are the VMs stored on the ESXi 6 hosts' local storage, or on some shared storage to which you may be able to connect one of the ESXi 8 hosts too?

In case of local storage, you could -assuming sufficient bandwidth - try https://williamlam.com/2012/06/how-to-copy-vms-directly-between-esxi.html. I haven't tried this between v6 and v8, but it may be worth a try. It's basically the same as exporting/importing as an OVF, but will save time due to the direct copy.

Another option would be to restore the VMs from backup, which I assume you have.

André