yes Turbotax will fill in the 8606, the questions on non-deductible contributions, basis carryover, value of IRAs as of 12/31/25, and Roth conversion amounts etc all flow into 8606. But do review the outcome on this form and your 1040 line 4b. Hopefully no CPA required, 8606 is fairly straightforward to follow.
and yes to your assumption - the $300 does NOT affect your contribution for 2026 as you would have already recorded that against 2025 (make sure in your brokerage account it was designated as a 2025 contribution also otherwise your Form 5498s will be off - those usually come in May).
