Box 7 should have the IRA box checked. The SEP box is only for actual SEP contributions. Same for a recharacterized Roth contribution as a TIRA contribution. Note that recharacterizations cannot be done between SEP and non SEP contributions and since 2018 Roth conversions can no longer be recharacterized.
Making a TIRA contribution to a SEP IRA may increase the chance that the custodian will make an error and show a SEP contribution that indicates participation in an employer plan. That could affect the deduction for the TIRA contribution and create a mismatch between the 5498 and the tax return. Therefore, if you plan on making a TIRA contribution to a SEP IRA, highlight that contribution as NOT being a SEP contribution.