How to format latitude and Longitude labels to show only degrees with suffix without any decimal or minutes?

I would do this by selecting Decimal with Suffix for the format and at the bottom of the Map Grid Properties select 0 for Coordinate Precision.

In your grid properties first make sure to choose EPSG:4326 as CRS, then choose "custom" as coordinate format and click on the expression button next to it:

Now use this expression:

abs(@grid_number) ||  '°' || -- concat absolute grid number (without -) with °
if(@grid_number=0,'', -- if grid number is 0, dont use a suffix
    if(
        @grid_axis = 'x', -- determine which grid axis it is
        if(@grid_number<0,'W','E'), -- If grid number is negative and axis is x, use W, else E as suffix
        if(@grid_number<0,'S','N') -- If grid number is negative and axis is y, use S, else N as suffix
    )
)

Result: