Can I open an Individual 401k if I already maxed my Roth IRA?

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I am 57. I have:

Individual Brokerage account with a small amount of stocks/value in it.

Roth IRA with about $85k in it (I started really late!)

401k from my previous employer currently parked at Empower.

I cannot contribute to that 401k anymore, and my Roth 401k is maxed so looking for the best way to keep contributing with the best tax implications. These tax implications should cover:

Taxes on gains once I retire and need to sell off stocks/etfs for $.

Taxes on gains if I need to sell and need $ for emergency.

Also, what happens if I open an Individual 401k, buy a bunch of stocks in it, then later this year or sometime next year, I get a W2 job? What happens to the Individual 401k? Im assuming I cant have both Individual 401k and employer 401k. I like not having to answer to a boss, but really dont like not having a base salary. Doing side jobs is alot more fun when that is play money or invest money vs money I need to depend on.

If Individual 401k is the best way to go for me at this point, how do I set it up? I use Fidelity for my Roth and Individual brokerage accounts.