Yes, you can contribute more money to your solo 401(k) plan. The maximum contribution to your 401(k) is $58,000 and because you have contributed $19,500, you can still contribute additional funds to your 401(k).
Another option you might consider is to make a contribution to a traditional IRA. A contribution to a traditional IRA may reduce your taxable income however, because you are already covered by a retirement plan at work, income thresholds could become an issue which would prevent you from deducting your IRA contribution. You could still make the contribution to the traditional IRA; however, your contribution (if you went over the income thresholds) would be "non-deductible," and you would want to prepare Form 8606 along with your return so you could track you non-deductible contributions. TurboTax will prepare Form 8606 for you if required.
Here is link to traditional IRA income thresholds that you might find helpful. New income ranges for IRA eligibility in 2021
Perhaps for now, your best option may be to just increase your 401(k) contribution to the point where your tax liability is more consistent with your financial goals.
@futurestrader39
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