Tracking after-tax basis in a solo 401(k) has nothing to do with your individual tax return. It's the plan administrator's responsibility to track the amount of basis in the after-tax sub-account of the traditional 401(k), calculate the proportion of after-tax basis in any distribution from the solo 401(k) and and report in box 5 of Form 1099-R the amount of after-tax basis included in a distribution.
The reason that individuals must track basis in traditional IRAs is that such tracking is the individual's responsibility because the basis belongs the individual, not to any particular traditional IRA account owned by the individual. Unlike 401(k) plans, all of your traditional IRAs are treated in aggregate as one large traditional IRA.