Hi tsunamis,
If you can test the combination in person to confirm that it’s a good match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) and you also include suitable covers to protect the layers and the materials are good quality and durable materials then it could certainly be worth considering. If you buy the foam base and then add a topper afterwards it can be a little more risky or uncertain (see post #2 here).
There is also more about the most important parts of the “value” of a mattress purchase (which would also include a DIY mattress) that can help you make more meaningful comparisons between the mattresses and options that are available to you in post #13 here.
Phoenix