Thankfully this is there (well for the Garmin 305 it is). It is under the map in the box titled 'timing'
hopefully NOW in 2020 there is a way to add this? Any suggestions?
Yes, of course: open the Activities page in Garmin Connect Web, and it is right there in the chart.
And then, of course, it is also in the detailed pages of each individual activity. Speed is used at cycling, 1km or 1mil pace at run and walking, and 100m/100y pace at swimming.
If you filter the activities by category, using the buttons above the chart, you can adjust the visible columns by clicking the symbol on the right side of the chart header, and add the average speed, if it is missing for the concerned activity type.