Yeah would love an answer here too

When you sell shares, cost basis is reported to the IRS by the brokerage. The parent comment suggests that RH always reports the FIFO cost basis, i.e. price of the first shares you bought as opposed to the last shares. There are places on tax forms where you can adjust your cost basis up or down from the one reported by your brokerage, but (without doing any research into it) I doubt that "I don't want to use my brokerage's cost basis method" is a valid reason to use those.

I really have no idea, but it's a VERY new thing that brokerages even report this information. The 1099-B can't be more than a decade old. You used to have to track it all yourself. I suspect you can use your own cost basis method. (subject to rules; of course you have to be consistent)

That said, this is definitely not tax advice, and even if it's legal/allowable, you're almost certainly more likely to be audited if your numbers don't agree with what the brokerage reports.

This is why Form 8949 (column (f)) exists.