When you buy a pro-grade microphone and interface, you are investing in hardware that should last you a decade if you treat it well. Avoid the entry-level gear that breaks in a year and focus on industry-standard pieces that hold their value.
Wait for Prime Day to grab these items because tech companies often discount their older, perfectly capable interfaces when new versions launch. If you do not need the newest software integration, last year's model will serve you exactly as well for a fraction of the cost.
The 15-month intro window is a deadline, not free money. If you have a remaining balance after the 15 months pass, the card will start charging interest on whatever is left over.