Year-end tax planning for self-employed: Solo 401(k) vs SEP IRA and deductions

[Savannah24]

I'm a freelance consultant who had a much higher income this year due to a few large projects, and I'm realizing my usual approach to estimated tax payments won't be sufficient. I'm looking into more proactive tax planning strategies before year-end to mitigate a potentially large tax bill. For other self-employed professionals or small business owners, what deductions or retirement account contributions have you found most effective for reducing taxable income? I'm particularly interested in the nuances of a Solo 401(k) versus a SEP IRA given my variable income, and whether there are any legitimate home office or business expense deductions I might be overlooking. How do you work with an accountant to project your liability accurately, and are there any specific quarterly estimated tax strategies you use to avoid underpayment penalties?