There is nothing impermissible about making maximum permissible deductible contributions to a solo 401(k). It's a common way to reduce present taxable income with the understanding that this deferred income and the gains on it will eventually be taxable as ordinary income when distributed.
Keep in mind that distributions are taxable as ordinary income, so it generally makes sense to defer the taxable income only if you expect your marginal tax rate when eventually distributed to be lower. For those with good saving habits, that's often not the case, so someone presently in a low tax bracket such as yourselves might be better off deferring less income now and instead investing that money in capital investments where gains can enjoy taxation at long-term capital gains rates instead of at future ordinary income tax rates. Still, you might want to defer enough income to keep you in the AGI range that allows the maximum Retirement Savings Contributions Credit, which might preclude you from topping out your current tax bracket. With AGI below $46,000 for 2024, the retirement contributions that you made would typically make each of you eligible for a $1,000 tax credit on Form 8880. (If slightly above that AGI, it might make sense for some of the Roth IRA contribution(s) to be traditional IRA contribution(s) instead.)
(Note that there is no our Roth IRA. Roth IRAs are not joint accounts, they are owned by a single individual.)
