" (25% of net earnings up to cap of $57k less my other 401k contributions)"
That's incorrect. As a single-member LLC, the maximum SEP contribution is 20% of net earnings. Net earnings are your net profit on Schedule C minus the deductible portion of self-employment taxes up to the section 415(c) limit, $57,000 for 2020. It's unaffected by anything regarding the 401(k) at your other job.
If you change to a multi-member LLC, the LLC will need to file a partnership tax return and contributions made to the SEP plan under the partnership, with the contributions reported with code R in box 13 of the Schedule K-1 (Form 1605). The maximum contribution will still be 20% of net earnings, but net earnings will then be the amount reported with code A in box 14 of the Schedule K-1 minus the deductible portion of self-employment taxes. Furthermore, the SEP contributions for both partners must be the same percentage of net earnings (limited by the 415(c) limit). SEP contributions are employer contributions, not employee deferrals.
The partnership must establish the SEP plan and a SEP-IRA must be established for each partner to receive the SEP contributions. The SEP-IRA custodian reports SEP contributions on Form 5498, no other notification to the IRS is required. However, the SEP plan must be established using an approved agreement such as Form 5305-SEP, retained in the partnership's files.
(Although the maximum base rate is 25%, because the SEP contributions are employer contributions, the SEP contributions themselves reduce the amount which is multiplied by the maximum 25% base rate. The result is an adjusted maximum rate of 20%.)
