Does the IRS expect the FIFO/LIFO/HIFO methods to be applied to all the crypto assets owned by the tax subject (even those not used for trading at all), or is it applied within one exchange at the time?
the determination applies to all of your assets.
if you're using multiple exchanges and maintaining a separate long term stash, specific lot identification will be easier. it will probably provide the best tax benefits too.
Specific identification can be used as an accounting method for trading crypto assets. This means that taxpayers can select which units and lots of cryptocurrency they are selling at a given time as long as they can specifically identify them and support the cost basis of the units.
If you are unable to specifically identify your cryptocurrencies, you are to default to a first-in first-out (FIFO) basis.
https://cryptotrader.tax/blog/new-irs-cryptocurrency-tax-guidance