You can enable a quick switch gesture to switch between the last 2 opened apps by swiping the app away and continuing the swipe past the center of the screen using the following experimental dconf option:
dconf write /desktop/sailfish/experimental/quickAppToggleGesture true
See https://forum.sailfishos.org/t/quick-app-switch-gesture-finally-works-in-4-0-1-please-enable-it/ for more info. It can be a bit buggy and not sure if it will ever land in the OS.
I don’t think QtWebengine in the browser will ever happen. The gecko engine is continuously being updated and QtWebengine is a bit of a mess with the old Qt, still requires python2 and otherwise would add a lot of maintenance overhead. Also playing into that google monopoly is not that great. Similarly, RCS is a mess of a standard… I wouldn’t want to support it, but maybe at some point Sailfish will have to.