Everything looks right. If I understand correctly, the $449,000 in traditional IRAs on December 31, 2023 is the sum of $201,000 in one traditional IRA and $248,000 in a different traditional IRA.
Your spouse's inherited IRA has nothing to do with this.
Basis in your traditional IRAs is nondeductible contributions. Your basis in traditional IRAs on December 31, 2022 was zero if you never made any nondeductible contributions to your traditional IRAs prior to 2023.
I suspect that the $900 on your 401(k) statement indicated the dollar amount rolled over to the traditional IRA from the entire after-tax sub-account in the 401(k) which consisted of $400 of after-tax funds and $500 of taxable earnings. The amount in box 5 of the Form 1099-R, $400, is the amount of after-tax basis that rolled over to your traditional IRA which is transformed into nondeductible traditional IRA contributions.
I would be a little more specific in the explanation statement by indicating the actual amount, $400.
Regarding the $353 on line 14 of your 2023 Form 8606, yes, this amount carries forward to line 2 of the next Form 8606 that you file.
