I’d go with 3" of continous pour dunlop or talalay latex, 19 ILD. No topper cover (encasement) unless there is a lot of activity on the bed from kids, dogs, etc. Just throw a high quality, stretchable, fitted cover over the mattress and topper, a fitted sheet and you’re done. Depends on your weight but our 19 ILD 3" mattress topper works great for both me and my wife.
Check out the Cover thread. https://forum.mattressunderground.com/t/mattress-cover