I posted this a few hours ago on one of WCI's older Backdoor ROTH IRA posts before I realized we had a community forum right here! So here goes my first post!
First and foremost, thank you to WCI and this community. This thread is pure gold and epicness! I’ve read this post a few times and have browsed through most of the 1200 comments so forgive me if my situation below has been discussed before.
I am an optometrist who started a SIMPLE IRA for my doctor associates and support staff about 10 years ago. This made sense at the time because the SIMPLE IRA was low cost and low maintenance. I believe it still make sense today as the doctors all contribute but only a handful of my regular staff does. Anyway, thanks to max yearly contributions and the magic of compounding interest, I have a good chunk of my retirement in this tax-deferred bucket. My wife and I had also been contributing to our respective ROTH IRA’s until the income eligibility as a married couple did not allow us to. We also started saving money in a Vanguard taxable bucket.
From reading these posts and others like it, we believe we should also continue to grow our money in the tax-free bucket, aka ROTH IRA. My understanding is we can do this by doing a backdoor ROTH IRA as described in this post. But in order for this to make sense tax-wise and avoid the dreaded “pro-rata” hit, we need to rollover my SIMPLE IRA balance over to maybe a Solo 401(k) and make sure the SIMPLE IRA balance is $0 by 12/31 before I do the 1) $5500 tIRA contribution then 2) backdoor to ROTH IRA.
So after my long-winded set up, here are my questions:
1. Can I get an EIN (sole proprietor) separate from my original practice (S-corp) and open a Solo 401(k)? I believe I have to actually have earned income with this “new” company in order to open the Solo 401(k), so I’ll fill out some surveys or maybe moonlight at some of my colleague’s offices a day or two. The purpose is to have the Solo 401(k) to rollover all my SIMPLE IRA funds. From the posts above, Fidelity’s Solo 401(k) will allow this and I do meet the two year requirement to rollover my SIMPLE IRA to another tax-deferred account without penalty. Too bad Vanguard’s Solo 401(k) does accept this type of rollover as all my accounts are there.
2. Can I have both a SIMPLE IRA with my original practice AND a Solo 401(k) with my “new” sole proprietorship at the same time? I still plan on contributing/maxing out my SIMPLE IRA every year and then rolling over the money to my Solo 401(k) every year right before doing the backdoor tIRA to ROTH IRA method.
3. And if number 2 above is allowed, does my yearly tax-deferred contribution limits go up? Currently my SIMPLE IRA limits me to $12500 as an employee plus an additional up to employer 3% salary match. Now that I have the Solo 401(k) set up, does my contribution limit go up to $18500? Or even higher?
4. Or is everything wrong above and I should re-read this post and all the comments again?

Thanks,
KN