Is a backdoor ROTH IRA contribution still worth it if I have pre-tax IRAs that I can't rollover to a 401k yet?

I'd suggest you withdraw your $3k after-tax contribution. You can do this by 15 October with an extension or amended return. You'd pay tax on the gains, which should be minimal since you didn't invest the $3k.

Finally, make a new $6k posttax contribution. This, you must do by 15 April; extensions aren't allowed.

Aganju points out that the sequence of events is immaterial. And in any case, what I'd said was incorrect; any conversion or rollover you do in calendar 2021 can only show up in your 2021 tax return. So it looks like the only thing you can do to not trigger the pro-rata rule is to skip your 2020 contribution.

You'll have to either:

• live with it and file Form 8606 each year you contribute or Roth-convert posttax balances, or
• withdraw the 2020 contribution you've made and not make any posttax contributions without first clearing out the entire IRA's balance using a reverse 401(k) rollover / Roth conversion (and for the SIMPLE IRA, you'd have to wait two years from your first contribution).

There is no time or amount limit for conversions (except processing time between contributions and conversions, so maybe two days inbetween).

You can convert all your SEP IRAs and Traditional IRAs today (or any other day) if you want to (but consider the taxes that you will have to pay for it).

This is independant of pre-tax and / or post-tax contributions - post-tax conversions will be tax free; pre-tax conversions are taxable, but they are always considered ’merged’ by the IRS, no matter how clear you separate them, so any conversion is always a proportional mix.

Once you merged pre- and post-tax money, it’s merged, until you empty all your IRAs past a year change; and you need to keep track with form 8086 (or get double taxed).

I would keep the money in your traditional IRA and look into rolling your other IRAs into your current 401(k). Once you get that done, then you can freely convert your Traditional IRA, and only pay tax on the gains. As Aganju mentioned in their answer (which this is somewhat an extension of), there is no time limit on conversions. So you could leave this money in your traditional IRA for several years until it is advantageous to do the conversion, if necessary. If you really want to minimize your taxes, you could even roll over the earnings in your traditional IRA as well, and only leave the non-deductible basis, so there would be zero tax on the conversion.

If there is really no chance you'll be able to get your other IRA money into a 401(k), then you should do a withdrawal of excess contributions with your brokerage. But there is no rush because you have until you file your 2021 taxes, which should be at least a year away.