You can contribute to a Roth IRA in addition to a Solo 401(k), provided your income falls below the IRS limits. For 2025, the income phase-out ranges are:
•Single: $153,000–$168,000
•Married filing jointly: $228,000–$243,000

If your income is too high for a Roth IRA, you can still contribute via a backdoor Roth conversion.