leafletProxy to change map with fixed fillOpacity in addPolygons, Shiny - r

Question

I made a Shiny app with Year slider so that the map could update the value by Year with leafletProxy.
However, if I set fillOpacity less than 1 in addPolygons. The opacity from the next Year would overlay on the previous display. So one after one, it would fully be opaque. I could add clearShapes() after proxy %>% to overcome this, but the shape would be re-drawn and I could see the refresh.
With fillOpacity = 1, this would not be a problem, but the color is just too bright.
The first graph below is the initial view, and the second one is after sliding by one year.
Is there anyways to fix fillOpacity for each update without clearShapes()?
The parts of my codes for this are:
observe({
df <- mapSelected()
value <- as.numeric(displayValue())
proxy <- leafletProxy("mapPlot", session, data = df)
proxy %>%
addPolygons(fillColor = ~regionPal()(value), fill = TRUE,
fillOpacity = 0.6,
color = 'white', weight = 1, opacity = 0.8,
popup = paste(mapLevel(), ": ", df[[SAName()]], "<br>",
"ED counts: ", value),
highlightOptions = highlightOptions(color = 'white',
weight = 3,
opacity = 1,
bringToFront = FALSE))
})
output$mapPlot <- renderLeaflet({
leaflet() %>%
addTiles() %>%
fitBounds(lng1 = 140.99926, lng2 = 153.63873,
lat1 = -37.50508, lat2 = -28.15702)
})