So then you know that you CAN use an after-tax 401k after maxing out your Roth 401k/traditional 401k space, if offered by your plan.

If your plan allows it, you can contribute to an after-tax 401k above the limit for traditional and Roth, up to the total of $66,000 for all employee+employer contributions in 2023. To be clear, an after-tax 401k is different from a Roth.

I think you missed the thrust of my question regarding company true-ups.

Let's say your company will match dollar for dollar up to 3% of your salary. You contribute 6% of your salary every paycheck, but you max out your 401k by the end of June and dont contribute the second half of the year.

If you work for a company that does not True Up, they will match the 3% for the paychecks that you contribute, that's it. So by maxing contribution early you miss out on the full company match. This is an extreme example but with Fidelity's percentage method of contribution, it's easy to miss the match for the last paycheck.

But if your company will true up the match at the end of the year, it doesn't matter if you miss contribution weeks as long as you work there for the whole year and your contribution percentage is high enough. For my example, since you contributed 6% for half the year that is equivalent to 3% for the whole year, and the company will make up the missing match.

Two questions that you need to investigate & answer first:

(1) if your company has a 401k match, will they "true up" at the end of the year?

(2) Will your plan stop contributions when you reach the total? Or will they switch to an after-tax 401k?

You can officially pass on the income to your friend. If StubHub sends you a 1099-k, then you can submit a 1099-k or 1099-misc for his half of the income. Then you each report half the income and half the costs and pay your own taxes.

Your taxes would be your capital gains rate which depends on if you held onto the tickets for more than a year.