This is probably going to be an unpopular opinion, but here it is anyway. If it doesn’t come off with dish detergent and a scrub sponge, just call it a permanent seasoning and let it be. If scrubbing it like mad won’t make it come off, then it won’t come off if your food touches it.

Our old dishwasher and previous cats sped up the process with the one thing that kills hardwood floors.

You should try installing some good ole linoleum. We solved kitchen flooring in the 1860s but people need to install expensive floors that are worse in every way because… why exactly? I don’t know. I have hardwood floors that are 17 years old and they need to be replaced. Linoleum floors last as long as 40 years. I’m thinking of going old school.