Hi gyoung,
Welcome to the Mattress Forum! 
It’s good to see that you’ve read the tutorial.
Two of the most important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to choose a suitable mattress (either locally or online) that is the best “match” for you in terms of “comfort” and PPP (Posture and alignment, Pressure relief, and Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful quality/value comparisons between mattresses in terms of suitability (how well you will sleep), durability (how long you will sleep well), and the overall value of a mattress compared to your other finalists (based on all the parts of your personal value equation that are most important to you).
There is more general information about what I call “simplified choice mattresses” that are sold online in post #1 here and there are some comments about Saatva and Winkbeds and many of the other simplified choice mattresses in post #2 here in the same topic. Forum searches on Saatva and on Winkbeds (you can just click the links) will also bring up more comments and feedback about them as well. As you can see both of them include some lower quality/density materials that could be a weak link in their mattresses in terms of the durability and useful life of the mattress.
If you purchase a mattress that is too firm for you then a topper can certainly be a good way to add some additional softness and pressure relief that you may need as an alternative to returning the mattress for a refund and choosing a different mattress.
While there is always some risk and uncertainty involved in adding a topper if you haven’t tested the combination in person 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 you do decide to add a separate topper to a mattress then there is more information about choosing a topper in post #2 here and the topper guidelines it links to which along with a conversation with a reliable supplier that can provide you with good information about how their toppers compare to each other or to other toppers they are familiar with that are available on the market can help you use your sleeping experience as a reference point and guideline to help you choose the type, thickness, and firmness for a topper that has the least possible risk and the best chance for success. It also includes a link to a list of some of the better online sources for toppers I’m aware of and a link to the online suppliers that have good exchange/return policies as well.
If a mattress/topper combination is a good “match” for you in terms of "comfort and PPP then it also has 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 very 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 and a topper can also help extend the useful life of the mattress underneath it as well.
As you probably know from the guidelines here that are linked in the tutorial … I would avoid the major brands or any mattress that uses lower quality materials or where you aren’t able to find out the specifics of the materials in the mattress (see this article) and the chain stores or major retailers that specialize in them such as Macy’s and Mattress Firm. Denver mattress on the other hand makes some good quality/value choices although I would also avoid the major brands that they carry as well.
The better options or possibilities I’m aware of in and around the Denver/Boulder, CO areas (subject to making sure that any mattress you are considering meets the quality/value guidelines I linked earlier in this reply) are listed in post #2 here.
Phoenix