Hi kfsleepy,
You can see my comments about choosing a firmer mattress first with the intention of adding a softer topper later (where you can’t test the combination in person) in post #2 here.
In most cases I would avoid this approach because of the uncertainty involved with making two purchase choices instead of only one and choosing a topper that would be suitable in terms of thickness, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences) for a specific person on a specific mattress can sometimes be almost as difficult as choosing a mattress that doesn’t need a topper in the first place. I would generally focus on choosing a mattress that is likely to be a suitable match without a topper (unless you can test the combination in person or you are purchasing both online as a “set” that is designed to work together and they both have a good return/exchange policy) and then use the option to add a topper as a “backup” strategy in case your initial choice is too firm and doesn’t turn out as well as you hoped for rather than a “primary” strategy.
If you do decide to try the mattress/topper strategy then if the only issue with a mattress is that it is too firm and there are no soft spots or sagging in the mattress then a good quality topper can certainly be an effective way to add some additional softness, “comfort” and pressure relief to your sleeping system but the only way to know for certain whether a specific mattress/topper combination is a good “match” for you in terms of comfort, firmness, and PPP is based on your own careful testing or personal experience on the combination. If you can’t test the combination in person then there will always be always some risk and uncertainty involved in adding a topper because the specifics of the mattress itself along with your own body type, sleeping position, and preferences can affect which specific topper would be a suitable choice on any specific mattress.
If a mattress/topper combination turns out to be a good “match” for you in terms of “comfort”, firmness, and PPP then having a separate topper does have the advantage of being able to replace just the topper without replacing the entire mattress if it softens or breaks down before the upper foam layers in the mattress (which is often likely because the upper layers of a sleeping system tend to soften or break down before the deeper layers) or if your needs or preferences change over time. A suitable topper can also help extend the useful life of the mattress underneath it because it will absorb some of the compression forces that can soften or break down the top layers of the mattress underneath it over time.
Zinus is a Chinese manufacturer that makes a wide range of brands including Enso, Vivon, Spirit Sleep, Night Therapy, Keetsa and many others (see post #2 here). They tend to use lower quality/density and less durable materials in their mattresses than I would normally consider. A forum search on Zinus (you can just click the link) will also bring up more information about them as well.
They are all manufactured in China and shipped compressed to North America so I would also read post #6 here for more about Chinese imports and some of the additional uncertainty and risk that can be involved in purchasing them.
As Jeff mentioned … I would tend to avoid them.
Phoenix