Should I upgrade to DDR5 for my RTX 4090 at 1440p?

No. Your RTX 4090 is already CPU-bound at 1440p, not RAM-bound. Upgrading from DDR4-3200 to DDR5-6000 nets at best 3–5% in a few CPU-heavy titles, often within the margin of error. The $300+ for a new motherboard and RAM is better spent on a 4K monitor or a faster CPU later.