Hi Sleepyone,
This is a very common comment with latex because it’s very “point elastic” (meaning it can contour and compress in a specific area with less affect on the surrounding area) so you can sink in more deeply with the more concentrated weight of sitting vs the more evenly distributed weight of lying down. You are also in a weight range where you may benefit from 12" of latex (see post #14 here). This is also why it’s so important to test mattresses with both of you on the mattress in all your sleeping positions and for long enough for your muscles to fully relax so your testing can do a better job of predicting your real life sleeping experience.
While I don’t know the firmness of the Cazenovian topper … I would be cautious as well adding 3" of 19 ILD Talalay because you could be “overcompensating” by adding too much additional thickness in an ILD that low which could be risky in terms of alignment (more for you than for your wife).
Phoenix