AGI an filing status are not factors in determining the maximum SEP contribution. The maximum SEP contribution is 20% of net earnings from self employment or $58,000 (for 2021), whichever is less. Net earnings are net profit minus the deductible portion of self-employment taxes.
The $66,000 AGI threshold determines the deductibility of a regular traditional IRA contribution for someone filing single who is covered by a workplace retirement plan. Contributing to a SEP-IRA makes you covered by a workplace retirement plan.