I think that the best topper suggestions I can make are in post #2 here and in post #8 here which it links to.
Part of the challenge of choosing a topper is that how it feels and performs is not only dependent on the specific material itself (and there are dozens of varieties of memory foam each with different characteristics) but to a great degree on the mattress that it is used on. All the upper layers of a sleeping system (a topper and the mattress comfort layers) all affect and “modify” each other to different degrees.
You are in a weight range where either 4 or 5 lb memory foams would work well and not present a durability issue (I tend to start leaning towards firmer feeling, slower reaction, but more durable 5 lb memory foams when weights reach around 200 lbs or so) but all other things being equal I would tend towards higher density memory foams … particularly if they are more breathable like the Aerus … because they will be more “supportive” and not allow you to sink in quite as far.
Gel “beads” can also increase the support factor of memory foam (meaning it will resist compression more as you sink in deeper) but larger particle sizes can also reduce the durability of the foam structure. The Serta topper also doesn’t include the density/durability information about their topper but it appears to be in the 4 lb range.
Like all foams … memory foam responds to pressure but unlike other “fast response” types of foam they also respond to different degrees to heat, humidity, and time (they gradually relax or “creep” with constant pressure) which means that they can be more tricky to use and maintain good alignment over the course of the night than other types of foam because you can start off in good alignment but then over time end up sinking in too far. Because of this … “just enough” to provide the pressure relief you need is usually safer than choosing thicker toppers … particularly with memory foam. Thicker/softer toppers that you sink into more deeply will also tend to be warmer than thinner toppers where you don’t sink into insulating materials like memory foam as deeply. Memory foam as a category tends to be the warmest of the three types of foam (polyfoam, memory foam, latex). Gel memory foam is a category of memory foam which uses either thermally conductive gels or phase change gels to help with temperature regulation and/or “support” but these both tend to have shorter term cooling benefits than foams that allow higher degrees of airflow. There are also many other factors involved in the sleeping temperature of a mattress besides the foams used as you can see in post #2 here and post #29 here.
Your “range” of “between soft to firm” is very wide ad these would be towards softer but if you are confident that you need “a fair bit” tending towards “a lot” of extra pressure relief rather than a thinner topper which would be less “risky” but would also not provide the same degree of additional pressure relief, and your mattress is quite firm and has fairly thin comfort layers and is evenly supportive (no dips or soft spots), then these could make a good choice. Between the two of them I would personally tend towards the higher density 5 lb Aerus which is both breathable (ventilation regulates temperature over the course of the night rather than many gels which only affect temperature for a shorter period of time until temperatures equalize) has a higher density, and doesn’t have larger gel particles which may degrade the foam faster.
One of the advantages of dealing with the big box stores is that you can experiment at low risk. With all the variables involved in different people, circumstances, and different types of foam … your own real life experience and experimentation will always be more accurate than any “theory at a distance” and their return policy allows for this. The disadvantage is that they know very little about the mattress or bedding products they sell or their suitability for different uses so they can’t provide any good advice, information, or descriptions about the specifics of what they sell or how they may compare to other options. The advantage of dealing with retailers or merchants that have much more knowledge (such as those that are listed in post #4 here) is that they can provide much more accurate and detailed information and guidance about the specific properties of the materials and toppers that they sell, and how they compare to other options on the market. Their disadvantage is that they often either don’t allow returns on “personal items” or returns can be more costly.
So if you are in experimental mode and the thickness of the topper in combination with the firmness/softness of your mattress matches what you are looking to accomplish (based on the more subjective guidelines in the topper guidelines I linked) … then I think the Aerus would make a good low risk choice.