Can I still open a Solo 401(k) if I already contributed to a SEP-IRA this year?

It can get complicated. You can technically have both a SEP-IRA and a Solo 401(k), but the employer contributions to both count toward the same overall defined-contribution limit the IRS sets each year, so you can't double up. Many providers set up a SEP using the older Form 5305-SEP, which doesn't allow you to also maintain another qualified plan for the same year. If you want the higher savings power of a Solo 401(k), it's usually cleaner to switch fully rather than run both. Because the interaction of limits and plan documents is technical, confirm with a tax pro before mid-year switches. The IRS covers plan coordination in Publication 560.