Should I upgrade my 2019 iMac from macOS Ventura to Sequoia?

If it's perfectly fine with Ventura, why update? Ventura is still receiving security updates, so security isn't really a concern. Your iMac is Intel, so Ventura will probably work best. You should think about update it at least to Sonoma next year, after the realese of macOS 16 (at that time, Ventura will stop receiving security updates).

Only you can make that decision. Ventura is a perfectly fine version of macOS. IMHO if it's running great with no problems why would you take the chance on upgrading now?

I suggest considering whether or not you need any of the new features in Seqoia before deciding if an upgrade is worthwhile.