Hi happycat,
I don’t think that synthetic latex is a “bad” material at all and I would consider it to be a superior material to most polyfoam. In comparison to other types of latex with a higher natural content it doesn’t have the same degree of desirable qualities so in the “latex” category it is the lowest quality and cost but it would still be good quality compared to most polyfoam. You can read more about some of the technical differences between SBR (synthetic latex) and NR (natural rubber latex) in post #2 here.
I think your mattress would make a good firm base for a topper and I personally wouldn’t start all over again and “throw away” a perfectly good mattress that just needs some extra thickness/softness on top.
There are some topper guidelines and sources in post #2 here (and the posts it links to) so you can use your own personal experience to decide on the thickness and softness you would do best with. If you add a blended or natural Dunlop or Talalay topper then most of what you feel would be the topper and you would still have the firmer support of the synthetic latex below it and I think it would be a good choice. It will also be significantly less than the $1000 you are mentioning.
Phoenix