A sharp new lens changes everything, and if you buy the right glass now, it should stay in your bag for the next decade. You want to buy this once and skip the upgrade cycle that just burns cash on gear you will outgrow in a year.
Spring is the sweet spot because manufacturers run trade-in events right as new models drop. That is when last year's top-tier lens often hits the sweet spot on price, giving you pro results without paying the brand-new tax.
Remember that the 0% intro APR for 15 months on purchases and balance transfers is a hard deadline, not free money, and any remaining balance after 15 months will get hit with standard interest.