Thermal Printers

I currently own a few Zebra GK420d printers. I have the darkness maxed out but would like darker. We print 800 plus a week on them. We use them for more than just FBA Sku labels. Anyone recommend a thermal printer that prints darker?

I have seen a few Zebra printers that are much bigger but I don’t want to pay $1000 for one that does not print much darker than these.

Thanks.