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Hello John,

The good news is that SEP-IRAs and 401(k) plans are wholly independent of each other provided that you own the SEP-IRA as a self-employed individual and the 401(k) plan is maintained through your employer. That means that you can make the maximum contributions to both your SEP-IRA and a 401(k) plan maintained through your employer.  However, the maximum contribution to each retirement plan is subject to two distinct limits:

1. For SEP-IRAs owned by self-employed individuals, the maximum contribution such self-employed individuals can make into their own accounts as of January 1, 2023 is the lesser of 20% of net earnings from self-employment or $66,000.  Given that you will need to know your net earnings from self-employment prior to making the contribution, you have until the due date of the tax return (including the extended due date if you timely file an extension) to make the contribution.

2. For 401(k) plans maintained by your employer(s), the maximum amount of contributions that you can defer pre-tax (through payroll deductions) is $22,500 for 2023. However, that maximum is an aggregate number that applies when considering all 401(k) plans in which you participated throughout the year. Therefore, you will need to make sure that you did not already make $22,500 of pre-tax contributions to your 401(k) plan maintained by your old employer for 2023 before making additional contributions into the 401(k) plan with your new employer. If you have not reached that $22,500 limit, then you can continue to make 401(k) contributions into the 401(k) plan of your new employer until you have reached that limit.

See the following resources for more information:
SEP Plan FAQs

Self-Employed Individuals – Calculating Your Own Retirement-Plan Contribution and Deduction

Retirement Topics - 401(k) and Profit-Sharing Plan Contribution Limits

Finally, with regard to your question about Restricted Stock Units (RSUs), they are essentially another form of compensation. Instead of getting paid in cash, you are paid in shares of stock. Furthermore, you typically have to meet Years of Service and/or other performance targets before you can obtain vested ownership of the shares of stock (hence the term "Restricted"). You will not realize a taxable event until the day upon which the RSUs you are initially granted vest. Therefore, you do not need to plan for anything regarding the RSUs at this moment because it typically takes some time for the RSUs to vest. 

When RSUs vest, the fair market value of the shares of stock on the vest date will be included in your income and reported on your W-2 for that year. Most employers, allow you to use some of the shares to cover the tax liability associated with the income inclusion (essentially selling enough shares for cash to cover the tax withholding on the income).  So, I hope that is enough information to quell your concerns at this moment.

Please feel free to look through the following resources for more information:

What are restricted stock units (RSUs) and how do I report them?

How to Report RSUs or Stock Grants on Your Tax Return

Good luck at your new job!