After thinking about it too much, I think there's one use for a Roth IRA in your situation: bond funds. Think about it - your massive capital loss has no impact on bond income, because it's treated as ordinary income. You could benefit from putting bond index funds into a Roth IRA (paying the tax once when you contribute). That helps you avoid paying tax as bond funds compound, with the added bonus that Roth IRA won't be taxed in the future.
If your assumption is correct, that you will never have more capital gains than your capital losses, then stock index funds belong in taxable. When you realize short-term or long-term capital gains, you wind up paying $0 in tax. If you put the stock index funds in a Traditional IRA instead, you pay ordinary income tax upon withdrawal. So taxable makes sense for your case.
Note that Roth IRA space for bonds has one additional advantage. Right now you consider the capital losses massive, and you think cars cost $30,000. But decades from now when that same car costs $100,000 owing to inflation, your capital losses might not look so massive. If you do run out of capital losses, you could appreciate having some space to put investments in a Roth IRA.