Hi JohnnyO,
You’ve probably read this but just in case some topper guidelines are here.
From the sounds of it you are in the range of a touch or a little or maybe a bit more so your idea of 1" - 1.5" seems reasonable to me in the same density or if you go thicker then using more supportive materials.
The choice between memory foam and latex is really a preference issue depending on how you feel about slow response vs fast response and how comfortable you are with each and the difference in feel between them. With a topper that thin you will feel partly the topper and partly the foams below it though so if you do use memory foam you won’t sink into the foam as much as you would if you had thicker layers of memory foam and you will feel some of the resilience of the foam below.
If you were to go with latex in the 2" range then around 24 ILD would probably be suitable. It will be more supportive than 4 lb memory foam but also different.
You may have been OK with of 5 lb memory foam (although it will typically feel a little firmer than 4 lb when you move especially) so if you stay with 4 lb then a little thinner would probably be good.
You could also try slow recovery latex which comes in 21 ILD and has a bit of the feel of both.
Walmart probably has the biggest selection of toppers out of the big box stores but some of them come from Wayfair or other vendors which have different return policies.
One of the links in the component post goes to this post which has links to lots of thinner memory foam toppers but again check return policies if that’s important to you.
Phoenix