that's correct, think of backdoor Roth as 2 separate transactions - (1) the non-deductible contribution; (2) a Roth conversion. You can backdate the contribution to prior tax year, but the conversion will apply to the current calendar year - so you'll get a 1099-R next year for the $300.
when you file for 2025 you would put in a $7000 contribution and also follow the questions about how much of it was contributed this year for 2025. your 1099-R will show a conversion for $6700 on the 8606, the end result should be a tax-free conversion of $6700 and you will have a basis carryover of $300 which will apply for your 2026 taxes.
the contributions are limited but the conversions are not, so say you do a backdoor this year for $7500 and do it all this year, then on your 8606 for 2026 you will have a basis carryover of $300 plus the $7500 contribution, for a total basis of $7800, and your 1099-R(s) will reflect a total Roth conversion of $7800 also.